The general information about frequency allocations (along with modulation types, permitted radiated power, channel widths, etc.) for radio frequencies used on the Railways can be found on the Offcom website.
If that wasn't detailled enough for your purposes then I would be wonderng why you wanted to know.
Is there any reason why an enthusiast should not listen in?
No practical reason other than that with the advent of digital communications it is largely becoming technically impossible for the amateur.
As recently is the 1980s it was possible to listen into the fire and police radios with nothing more exotic than a FM radio receiver - the world did not end and criminals did not have a field day.
Though they've alll moved to ever more secure systems- and obviously the railways are shifting to GSM-R now.
im just curious and wanted to know if they could be tuned into or not for my use only nothing more.
And don't drivers use the signal post phones for routine stuff.