There are some things on the WTR I just don't seem to be able to find. there are 20 channels allocated, the new frequencies for trains on the West Highland Line. Apparently the Helensburgh to Garelochhead section can be heard on 181.675.
"Following negotiation with OFCOM (the communications agency responsible for
radio spectrum management within the UK), Network Rail was allocated a total of 20
radio channels within a 2.4MHz section of Band III sub band 1 (180.4 - 182MHz) base
transmit paired with 188.4 - 190.8MHz mobile transmit) to support RETB operation.
The new frequency band is adjacent to the Band III frequencies that have been used
for NRN operation and thus the radio propagation characteristics will be broadly
similar."