Bi-Directional Running On Mainline

[Edwin_m]

Are these sorts of bidirectional running a (relatively) modern thing or did it exist in some form when such major lines were controlled by semaphore signals (i.e. outside of stations)?

If it ever happened the lines in question would have to have been treated as single lines, which until the 1960s would have involved exchanging tokens.  Might have been viable in the later years with some form of tokenless block, but by then track circuit block would have been the preferred solution if bi-directional running was necessary.