- Why is Sheffield platform 2c numbered as such?
It wasn't always a passenger platform, being originally a siding or "Dock" as it was shown on mechanical-era box diagrams that I have seen. It was provided with trap points and a shunting signal controlling the exit. It remained a dock when Sheffield PSB was brought into use in the 1970s, but according to a route learning guide that I have, the exit points (without traps) were controlled by ground frame, presumably released by the PSB. I imagine that this was done as part of the 1970s resignalling scheme. At that time, the main signals on approach in the down direction (S79 on "A" line" and S81 on "B" line) did not read into the dock. Their respective subs read to subs 88 and 89 respectively, which were released by the ground frame for shunting movements into the dock.
Some time later (I don't have a date) the dock was converted to a passenger platform. The ground frame was removed, motor points installed and a full exit signal (S105) provided. I can only agree with swt_passenger above regarding the reason why it was not given the number 3 and subsequent platforms renumbered; extensive modifications would have been required to the signalling system.
For what it is worth, at one time, what is now no.8 platform was originally no.9 at the south end and no.8 at the north end, the dividing point being the kink in the platform that is still there. There was a separate line from the north end to platform 9, bypassing platform 8. I believe that this was also removed during the 1970s resignalling, though to the best of my memory the full platform had previously been renumbered to platform 8 in the mechanical era. I stand to be corrected though!