I'm not aware of any legal limit. There are PIXC limits (Passengers in Excess of Capacity) which are used for franchise monitoring purposes (not sure if used for all TOCs or just those serving London) but as you say, how would you enforce a legal limit? Assuming no extra trains were forthcoming from FGW/DfT you'd end up turning more passengers away than currently - if you could somehow prevent them from boarding the train. They'd probably just force their way on anyway and understandably so if the next train is in an hour's time. And it's hardly fair asking the train Conductor to be the equivalent of a bouncer.
I think the only way to enforce a legal limit would be to have a 'no standing' rule combined with compulsary seat reservations. But seat reservations aren't practical for commuters who don't always know what train they're going to be getting. And what do you do when a train gets cancelled and the passengers have to try squeeze onto the next train. "Sorry we've exceeded the legal limit. You're not allowed on". Sounds like a recipe for a riot...

On a related theme the calls for train passengers to wear seat belts come up against similar problems. By definition any standing passengers wouldn't have a seat belt. If they were involved in a train accident you can bet that some of them would try to sue the train company for not providing them with a seat belt. In order to head off this possibility the train companies would have to run 'no standing' trains and the only way to do that is to have compulsary seat reservations. Possibly acceptable on Intercity journies but certainly not on local commuter trains.

Legal limits for standing passengers? The phrase "don't go there" springs to mind.   





     

Thanks, Steve and Ruth.  I guess the answer is ... (b) - it would be impractical.   Reasons / logic noted and agreed, effect on the two-many standees' safety also noted. But still many times safer than going by road