A few for the Northern Line

City and South London Railway stations Oval - Euston City

The platforms were originally 200' long. In most cases the site of the
original platform is easily seen as:
a) it is often the only straight part of the platform
(all the original platforms were straight)
b) the original station tunnel is a different shape (narrower)
c) many of the original platform ceilings still have their tiles showing
(painted over)
d) where there is work going on, the original brickwork may be seen
(the original platform tunnels were built out of brick, not iron
segments)

Kennington City NB
The wide tunnel mouth at the north of the platform. The track and platform
used to be on opposite sides. The station tunnel was extended southwards
and the track swapped. The wide tunnel mouth was necessary to get the
track back in line.

Elephant & Castle NB
As for Kennington, except that the wide tunnel mouth is at the south end
of the platform.

Stockwell
Although a bit hard to see from the platform, the original island platform
was where the crossover now is. There are still tiles on the ceiling of
the crossover. The original tiled exit is through a (by these days) narrow
passage to the left, just as the train enters the crossover area. It's
blocked off after about eight feet by a bulkhead door.

Moorgate SB
The stub at the south end of the platform where the engine siding used to
be.

Roger