"Richard J." wrote in message ...

Unprotected 3rd rail would not be allowed today on an entirely new
railway, but that has nothing to do with mixing tube and mainline, both
of which use the same technology at Richmond, except that LU is 4-rail
rather than 3-rail.

I never said that it did. Its to do with what happens to the different
types of trains if they collide with each other. Tube trains usually come
off worse because the buffer beam of the mainline train hits the tube train
on its body rather than its buffers. Not sure what the rules are for full
size tube trains such as A,C and D stock sharing mainline track however.
Anyone know?

B2003