Oh, Crossrail: don’t know where you’re going, don’t know where you’re coming from.

By now, we all pretty much thought we knew which route London’s new regional railway line was going to take: from Heathrow and Reading in the west, to Shenfield and Abbey Wood in the east. But today, transport secretary Patrick McLoughlin announced that the route could be extended again: from a junction at Old Oak Common, west of Paddington, along what’s currently a freight route to the West Coast Main Line (WCML), which runs into Euston.

Which services this new branch would take over is not entirely clear. In the past, when a Crossrail extension following this route was discussed, Milton Keynes was the destination, but in today’s announcement the town is conspicuous by its absence. Instead we get references to the Hertfordshire commuter towns of Watford, Hemel Hempstead and Tring.

That suggests Crossrail would take over the two services each hour which currently run as far as the latter station, about 30 miles from central London. (By way of comparison, Shenfield is around 20 miles out; Reading is nearly 40.) Here’s a map of where we think the line might go, with the new section marked in red.

 

There are two very good reasons why it might make sense for Crossrail to take over a few WCML services. The one cited by McLoughlin is that Euston will be the terminus of High Speed 2, the new fast railway line to the north. It’ll be rebuilt before that happens, but still, the station could do with all the spare capacity it can get, so shoving a few suburban services onto what is effectively a big tube line would help.The other reason to think this is a good idea is that the current plan is a little unbalanced. The metro-style line from Liverpool Street to Shenfield is one of the busiest main lines in London; Canary Wharf will get heavy traffic, too, so between them the eastern branches of Crossrail will get 24 trains per hour. As it stands, the western ones, which get less traffic, only need 10.

No final decision has yet been made about the extension: that’ll depend on both its cost, and on its impact on the existing project timetable. But, if you’re spending £16bn on a new railway line anyway, you might as well send the extra trains somewhere.