"Bob Wood" wrote in message
...
It might be obvious but not to me. What is the advantage of having all
the
units the same way round during the closure of the loop?

Basically, the problem arises because a 73ts will *NOT* couple A to A or D
to D.

Half of the single-ended units were built to face Heathrow whilst half were
built to face Cockfosters.

For example, you can have:
114-514-314 + 315-515-115 (D to A)
203-603-403 + 404-604-204 (A to D)
888-688-889 + 301-501-101 (D to A)

You can not have:-
114-515-314 + 312-512-112 (D to D)
895-694-894 + 892-692-893 (A to A)

By having all the units facing the right way (I've no idea if this is the
plan - Steve?!), as they were built to do, it guarantees that you have a
balance of A- and D-facing units, and generally makes it easy for the depot
staff because any unit can couple to any unit.

If you have some A-facing units facing Cockfosters and some facing Heathrow,
you have a situation where you can choose two units which are otherwise
correct for a train (i.e. a cab at each end) but which cannot couple to each
other. It is much simpler for depot staff if this situation is avoided.

The problem does not arise on the Northern or Central Lines because all
units can be coupled to each other. The reason this is not done on all stock
is because it requires the electrical wiring to be duplicated through the
coupling (couplers are either left or right handed, IYSWIM), which is more
expensive and more prone to failure.

It is not simply a matter of having line maps facing the right way, though
this is a useful spin-off as far as the passenger is concerned.