In message , rail
writes

Yes, there's a very low bridge[1] in Ryde that requires the use small stock,
hence the reason for choosing tube stock in the first place.

[1] And possibly others elsewhere on the system.

If it was merely a bridge, it could be rebuilt. The difficulty is Ryde
tunnel, which is almost a quarter of a mile in length and prone to
flooding. That wasn't a great problem in the days of steam, but when the
system was electrified, the only way to avoid constant short circuiting
was to raise the base of the tunnel by about a foot, hence the very
restricted headroom.
--
Paul Terry