I supose a train say a 142 for example could become dagerousley overcrowded when its that heavy that it wont move and the engine is about to blow !

Yeah, but is there a figure for it? Would, for example, an insurance company say that 120 people in a carriage is acceptable, but 121 isn't?

AFAIAA, unlike a bus there is no limit for the number of people allowed on a train. I've driven a train that was so full it stalled halfway up a hill. Fortunately a reboot brought it back to life and it struggled to the top.

Although a severely crowded train train doesn't cause a big worry for the TOCs, seriously crowded platforms do. There will come a point where the station staff close off the barriers/gatelines to prevent anymore customers enterring the station. This happens regularly at London Victoria where a beeping alarm warns passengers coming off Southern & Southeastern platforms that the LUL entrances are being temporarily closed due to overcrowding down on the tube platforms. Most passengers are used to it and just wait patiently - it's part of living on an overcrowded island.

That's intersting. About 5 years ago I was sitting on a fairly overcrowed HST train waiting at Paddington when the "train manager" made an annoucement before departure time that the train was overloaded and he wouldn't let the train depart until some people got off.

I did get off, but perhaps I should have been a bit smarter and demand an upgrade to first class and free FGW coffee in compensation.

I have ocassionally wondered how he had decided the train was overloaded, especialy as I have travelled on many worse overfilled trains on this route. Perhaps one of the power cars wasn't working - so the train would have had no chance of getting out of Paddington against the gradient and round the curve

I wonder what his criteria was. Under Thameslink there was a local instruction that when trains out of Brighton were diverted via Lewes/Keymer Jn the driver had to ensure that he was able to move through the train to the far end in order to do the reversing shunt at Lewes (you can't go outside the train as there's no cess to get down on). Since you only take this route when the mainline is stuffed you can be sure the train would be utterly packed. The instruction stated that the driver must tell enough people to leave the train at Brighton to ensure that it was possible for him to get through. Fortunately I never had to try this one out, as I really wouldn't want to tell someone who had been waiting for hours that they must get off whilst others could stay on.

Yes I suspect that there was probably some underlying issue/problem that he didn't let on about.