Crossrail 2

Crossrail2 is a project for a new cross-London railway running roughly from the south-west to the north-east. It is not a vanity scheme in itself - on the contrary, it is a long-overdue construction of a project first put forward in 1904 and from time to time ever since. It links together existing routes which at present running to Waterloo, by diverting them into an underground route northwards to Seven Sisters and Tottenham. Amongst other benefits, this will reduce pressure on congested Waterloo.

There are questions, nevertheless. From the south-west, four separate routes converge into one, which is always a potential source of operating difficulties. At the northern end, the line splits into two, which means that some of the trains arriving from the south will have to turn back before they get to the end of the route, probably at Tottenham or Seven Sisters.

More seriously, should it be built as a full-scale railway with overhead electrification, or as a tube? The former, with 6 metre diameter tunnels, involves excavating three times as much material as a 3.5 metre diameter tube.

A related issue is the choice of electrification. The routes from the south-west are already electrified with a 750V DC conductor rail supply. If this were continued throughout the length of the tunnels, the latter could be constructed to a 5 metre diameter instead of 6 metres, ie only 70% of the excavation volume. A conductor rail DC system would also avoid having to fit the trains with heavy transformers, as well as providing greater reliability and reduced maintenance costs, since conductor rail electrification is almost a fit-and-forget arrangement.

The argument usually given for 25KV electrification is that it is more energy-efficient, which is true over long distances where transmission loss becomes an issue at low voltages due to the much higher currents. However, Crossrail 2 is a short-distance railway and transmission losses are not going to be so significant.