I tend not to board excessively overcrowded trains, I prefer to wait for a following, quieter, service where possible.
One of the worst I have had no choice but to endure, as I had a booked reservation on a later train from Birmingham, was a single Crosscountry class 220 between Leamington Spa and Birmingham during snow disruption the Sunday before Christmas in December 2010, when very few trains on the Chiltern line were getting through and we had already been turfed off a Chiltern 165 which had struggled bravely on from Banbury as far as Leamington. The raging hangover didn't help any.
To be fair, the cramped conditions are often little different on regular Crosscountry services on the Birmingham - Stafford and Derby axes (probably others too, but the northbound legs are the ones I am familiar with), and not just at peak times: I recall once having to catch an overcrowded lunchtime Crosscountry service from Birmingham when heading to Newcastle one Friday or Saturday, with the added complexity that I was wielding a bass guitar and, for reasons I cannot clearly recall, more luggage than my 'travel light' ethos would usually countenance. I had a seat reservation, but there was no chance that I was able to get anywhere near it.
An honourable mention should also be made of the late morning Pacer between Sunderland and Newcastle that was so overloaded that it simply ground to a halt and stuck on the tight curve in Gateshead. The resigned tone of the guard in announcing the issue and the dexterity that the driver demonstrated in eventually negotiating his charge out of the bend suggests that this is far from the first time that this has happened, and single class 142 operated services between Sunderland and Newcastle are often full and standing even off peak with no major local football engagements on.
Having recently reacquainted myself with the North Transpennine corridor, I do pity those who have to regularly traverse the core section between York/Leeds and Manchester/Liverpool: It seems that all but the most peripheral of late night services, on any day of the week, suffer from some degree of overcrowding and I am quite familiar with standing in the large vestibule by the disabled toilet as the only option. The new trains can't come soon enough, though I do wonder for how many years even they'll prove sufficient to cater for passenger demand over the route.