Footbridge at terminus stations.

[roundhouse]

Plymouth Friary had a footbridge to the east end of the canopies

http://www.disused-stations.org.uk/p/plymouth_friary/

 

It was removed in later years.

[Zomboid]

I doubt if the datum point for mileages would ever be moved once set as you'd then have to revise the mileage for every bridge and structure on the entire railway. Any railway historians here may be able to elaborate but ISTR that the mileages on bridge plates etc. beyond Taunton are via Bristol not the later shorter route via Newbury and Westbury.

The Uckfield line was redone at some point, it was originally a bit of a mix with much of it being measured via Tunbridge Wells West.

But on the other hand, the entire former LSWR is still measured from the former junction with the Charing Cross lines at Waterloo East, so the Waterloo buffers are at about 5 chains. I guess they must have remeasured at some point, since their original London station was at Nine Elms.