The DfT set a limit on the data allowance that it deemed sufficient for browsing and email purposes, and reckoned that 50MB would be more than enough.
With much of the rail network staggering along on an infrastructure designed for the 19th century and earlier, capping based on web page sizes from the last century seems eminently sensible.
As of 09/07/18, the median desktop page consumes 1709.4KB while it appears on your browser, and so-called mobile pages take 1565.8KB, so it doesn't take long to burn through that 50MB!
The DfT then left it to the train operators to decide what should happen once that allowance has been burned through, and throttling is the name of the game.
Passengers on Southern trains will see their bandwidth drop to a 1990s level of 0.5Mbps.
Clearly, the roaming rules occasionally used by some mobile operators also apply to trains.
Taxi, anyone?