Crewe North Jn 'VOL' Meaning?

[elltrain3]

Hello all,
I visited the Crewe Heritage Centre on Saturday to visit Crewe North Jn S.B. and got into a conversation about the old (1940s) diagrams, and I understood most of the details (E.D.B. - Electric Depression Bar) etc
But On the end of all the bay Platforms (Pictured Below) "VOL" is written (twice in the case of B7 & 8, and I'm not sure as to the abbreviation meaning and after a search around drew no results I thought I'd ask here in case anyone has a better idea of what it is!

Thanks In Advance!

 

[Railsigns]

VOL stands for "vehicle on line", a switch near the track that operates an indicator in the signal box to let the signalman know there's a vehicle standing there. Here's a photo of one:

 

[elltrain3]

Ah, good to know! Thanks