As a train driver, it is not illegal for them to travel by themselves, but the railway companies WILL NOT keep an eye on them, its at your risk.
Their are not always Guards on trains as most train {especially down south} are driver only operated, it is possible to sit them behind the drivers compartment, but theirs no guarantee he could check on them every so often, theirs to many things that can go wrong on the railways {trust me} as other AB's have already pointed out, a coach journey would be much safer & if one of the boys has got a mobile phone that would be of benefit for peace of mind foyou and your ex, by all means contact your train operator, but i would be VERY surprised if they took responsibility
Good luck