This is Virgin's stated policy on how many people it will allow on a train.

"Virgin trains admit overcrowding safety measure is that "train can't move"
By Fraser Clarke - August 26, 2016 0 87
VIRGIN trains have admitted their safety measure against overcrowding is that the train is too heavy to move.
In a series of tweets with customers, the company was asked to explain its upper limit for passengers.
The company eventually admitted there was no limit on the number of people it would squeeze into a carriage, adding: "If the train reaches a certain overall weight then the train would come to a stop."
Virgin statement
Jim Carroll