I remember travelling on a ridiculously overcrowded train on a bank holiday Friday evening from London to Plymouth in the UK about 10 years ago, a journey of about 3 and a half to 4 hours. I wrote and complained to the train operator at the time and they said they there was no legal maximums on the amount of passengers that could be carried, so tough. If you've ever travelled by tube at rush hour in central London, scores of people are squashed into a single carriage; you'd be waiting a long time if you wanted a comfortable journey.
You don't say how the bang to the head occured. Was it as a result of some sharp stop that resulted in them tumbling? When you say "liable for injuries", is your inquiry about rectifying the medical condition that occured or seeking compensation over and above this?