Virgin Trains said that four toilets are being taken out of service every day as a result of people using them incorrectly.
This meant 18,000 lost toilet hours every year with a repair bill of more than £180,000.
The bra is not the only strange item to be found down the toilet on the operator's Pendolino trains - with glasses, wedding rings, nappies and a football scarf also causing plumbing problems.