What is... Crossrail?

Crossrail is a new underground railway line being built in central London: 26 miles of tunnel will link Paddington in the west to Liverpool Street and Canary Wharf in the east, with connections to existing lines at either end permitting trains to Reading, Heathrow, Shenfield and Abbey Wood. Construction is costing £15 billion, and the line is expected to open in earnest in 2019.  Here's a map:

© Crossrail Ltd 2014

(You can see more maps on the Crossrail website.)

Note that I deliberately didn't capitalise "underground" above: Crossrail is not (just) a new London Underground (LU) line, like the Victoria line in 1968 or the Jubilee line in 1977 (extended in 1999). For many practical purposes, it will end up being a lot like an LU line; but there are three key differences between Crossrail and LU:

1. the stations will be further apart, making it more like an "express" underground line;
2. Crossrail will extend much further out into the suburbs;
3. the tunnels and the trains will be larger (the same size as mainline trains).

Let's start with the first major difference, the distance between the stations. Particularly in the central tunnel, the line is best described as an "express" version of the Central line, and will bring substantial relief to long-suffering Central line commuters. While express underground lines are a radical new concept for London, they're a common feature in many other cities, most notably on the New York Subway.

Between Ealing Broadway in the west and Stratford in the east, the Central line has 20 stops; Crossrail will have just 7 in the same stretch. They are Acton Main Line, Paddington, Bond Street, Tottenham Court Road, Farringdon, Liverpool Street and Whitechapel. While Bond Street, Tottenham Court Road and Liverpool Street lie on the Central line, the route deviates north to meet the Circle line at Paddington and Farringdon, both of which will provide major interchanges with mainline rail services. It also deviates to the south to meet the Overground and the District line in another major interchange at Whitechapel.

The presence of Acton Main Line in that list betrays the second major difference: Crossrail will extend much further out than any tube line: the line will be 85 miles from end to end. To do so, it will use the existing Great Western Main Line (GWML) between Paddington and Reading, including the branch to Heathrow, and the Great Eastern Main Line (GEML) between Stratford and Shenfield in the east. (The branch to Canary Wharf and Abbey Wood, though, will be all new.)

Why extend it so far outside London proper? Currently, suburban trains to Reading arrive at Paddington, and sit for up to half an hour before going back towards Reading. That takes up valuable platform space which could be used by other trains. By transferring those trains to Crossrail, the trains can continue through to Shenfield on the other side of London, and free up those platforms at Paddington and Liverpool Street for other services.

The principle isn't new: we already have Thameslink, running north-south through London (of which more in the next blogpost). But perhaps the best example is just over the channel in Paris: the RER (Réseau Express Régional) is a network of five lines criss-crossing the French capital, linking the suburbs to central Paris:

(from Wikipedia)

The RER lines connect suburban lines on either side of the French capital through a central tunnel. Indeed, RER Line A (in red) is uncannily similar to London's Crossrail: it connects western and eastern suburbs through a central tunnel, broadly parallel to Metro Line 1 but with many fewer stations. The RER has been vital in releasing capacity for medium- and long-distance trains to use Paris's six terminal stations like Gare du Nord and Gare de Lyon.

The same effect will happen at Paddington and Liverpool Street once Crossrail opens: rather than wasting as many as a third of the platforms at Paddington with local services, they will be removed to separate underground platforms and continue through to Liverpool Street and beyond, leaving more platforms at Paddington for long-distance services.

In order for that to be possible, we come to the third important difference between Crossrail and the parallel Central line: its tunnels will be the same size as mainline railway tunnels, and hence Crossrail's trains will be much bigger than tube trains.

London has long been hamstrung from being one of the pioneers of "tube" tunnels. The earliest LU lines - the modern-day Metropolitan, Circle, Hammersmith and City, and District lines - were built simply by digging up the road, laying down a railway line, and covering it over. This "cut-and-cover" method of construction was fairly easy, but it entailed colossal disruption to road traffic. Nonetheless, the tunnels were designed and sized for mainline trains, and the four "subsurface" lines remain by far the least claustrophobic part of the LU network.

Faced with the political impossibility of building more subsurface lines, attention turned from cut-and-cover tunnelling to the more novel method of boring a tunnel, without any need for direct access from above. The innovations of Brunel (senior and junior) in building the Thames Tunnel led to the Greathead shield, the first major design of tunnelling shield, and enabled tunnels to be bored through the soft clay of south London: without it, the tunnels would have collapsed under the weight of the soil above them before they could even be lined with iron or steel.

The City and South London Railway, now the Bank branch of the Northern line, was the world's first "tube" line with bored tunnels. It was also the first major railway to use electric traction, necessary if the deep tunnels were not to become filled with smoke and fumes. But the technology of the time allowed a tunnel diameter of just 10ft 2in (3.1m). Over the next two decades, improvements meant that the Central, Piccadilly and Bakerloo lines were all constructed to a common diameter of about 12ft (3.6m), with extensions to the Northern line following suit and requiring the hugely disruptive enlargement of the original C&SLR tunnels in 1923.

But there it stayed: any further enlargement of existing tunnels would have been (and would still be) effectively impossible - such is the dependence of London on its Underground that any lengthy closure of one of the lines usually causes chaos. The Jubilee line was essentially an offshoot of the Bakerloo line, so there wasn't much point building the new tunnels any larger since the old ones it had to run through were still only 12ft across.

However, the decision to build the Victoria line at the same diameter as the other four tube lines was motivated partly by a wish for compatibility, but mostly it was down to budgetary constraints (some would call it penny-pinching). Indeed, the whole Victoria line was built decidedly on the cheap and it suffers badly for it now, with stations too cramped and trains too small to truly cope with the volume of traffic.

Fortunately, the designers of Crossrail have learned their lessons: the Crossrail tunnels will be a comparatively giant 6.2m in diameter, giving tunnels with three times the cross-sectional area of the tube lines. Indeed, the impression is that nothing has been skimped from the budget at all: some of the stations will be absolutely huge, in stark contrast to the failings of the Victoria Line.

Canary Wharf station on the Jubilee line is famous for being almost cathedral-like in proportions; it was built out of a hollowed-out dock from the old docklands, and can quite happily absorb over 130,000 passengers a day. Canary Wharf Crossrail station, to be constructed from another hollowed-out dock, promises to be even bigger: the whole box will be 475m long - over a quarter of a mile!

Perhaps more important, though, is the upgrade work being undertaken at existing stations to ensure the new Crossrail services don't overwhelm the existing stations. Most notably, Tottenham Court Road station is already full to bursting at peak times with just the Northern and Central lines serving the station, and leaving it unaltered with the addition of Crossrail was not an option. So the station is being completely and painstakingly rebuilt, ensuring a total capacity for over 200,000 journeys per day once Crossrail is open.

From the outset, Crossrail will have 24 trains per hour (tph) in each direction through the central core in the rush-hours, made possible by automatic train operation (ATO). The parallel Central line has 33tph, but its trains have a much smaller cross-section and are only 130m long, compared to Crossrail's 200m-long mainline-sized trains. It is clear that Crossrail will undoubtedly do wonders for London's transport system, and its knock-on effects will reach all the way to Norwich and Penzance.

Nonetheless, there are some criticisms that I could make about Crossrail. Most importantly, having extolled the virtues of joining services on either side of the city together, there is something of a mismatch between the service level planned for the central core and the outer extremities: of the 24tph running through the central core, 14 per hour will only run as far west as Paddington, and only 10 will continue onwards towards Heathrow or Reading.

Partly this is due to lack of capacity on the GWML, and partly due to lack of demand; it's hard to see the GWML core from Paddington to Ealing, Southall and Hayes ever needing more than about 12tph in the near future. The ideal solution would really to be have another branch for the other trains to use west from Paddington; but while many ideas have been proposed, such as branches to Amersham, Kingston and Milton Keynes, none have yet come to fruition and the line will open with one main western trunk and two main eastern branches.

By contrast to the situation at Paddington, each of the two eastern branches - Shenfield and Abbey Wood - will get 12tph in the rush-hour. This will give Canary Wharf, an area that has been growing for decades, a much-needed capacity boost. The two branches will meet at Stepney Green Junction, which will be completely underground. Nonetheless, one could again argue that Abbey Wood is too close to central London to be an eastern terminus of Crossrail.

Fortunately, adding an extra western branch or extending the Abbey Wood branch can be done without the need for an underground junction, and is thus easy enough to add later on. In some ways it was more important to get the central core under construction and let the branches figure themselves out later; I cannot imagine Crossrail living past its first decade without any changes in the pattern of services, since the appearance of services on the ground always brings changes that cannot be predicted at the construction phase.

I could go on, and on, and on about Crossrail, for it is really quite fascinating; but ultimately we are only halfway to actually having a working railway line. So let me leave you with some links should you wish to read more about this epic project:

• London Reconnections is an excellent site covering all aspects of London transport; they have a number of much more detailed posts on Crossrail, but I would particularly recommend how quickly will Crossrail fill up?; more detail on the off-peak services; and this and this provide some fantastic pictures of the new station at Canary Wharf.
• The Crossrail website contains a wealth of information, including detailed maps and plans; perhaps most interestingly, though, is the history of how Crossrail came to be.
• The Transport for London website has some information about the rebuilding of Tottenham Court Road station, complete with a video walk-through.

Next post: What is... Thameslink?