Tom Anderson wrote:

We went over this a few weeks ago (search for "dull questions about
loading gauge"): once upon a time, there were speed restrictions on C
stock out there, due to some sort of gauge business. However, (a)
trains would have to slow down for the station starter anyway (no
idea what that means!) and (b) the business is now taken care of.

(a) is probably in reference to an approach-released station starter;
in a controlled area like Dagenham East (FG), the interlocking is
probably designed to clear the relevant starter after the platform
track circuit has been occupied for a short period. In an auto area, a
timer circuit is used.