Toilets on trains run by First TransPennine Express have generated a greater volume of complaints than any other rail company in the UK.
Facilities in TransPennine carriages were responsible for 2.7 per cent of all complaints made by its passengers in April-June 2015.
The figure is over a third higher than a year ago.
The companies with the next highest volume of complaints about toilets were CrossCountry and Virgin East Coast (both 2.2 per cent), according to provisional figures from the Office of Rail and Road (ORR).
First TransPennine Express operates services across the north of England and in central and southern Scotland.
It received a subsidy of £43.5 million from the Government in 2014-15, the equivalent of 2.3 pence for every kilometre travelled by one passenger.
Besides receiving the highest volume of complaints about toilets, the company also received the second highest volume of complaints about both the overall standard of facilities (18.6 per cent) and the helpfulness and attitude of staff on trains (5.2 per cent).
But Transpennine enjoyed one of the lowest scores for complaints about punctuality and reliability of its services, however: 12.6 per cent.
This compares with 72.6 per cent for Chiltern Railways – the largest number for any operating company in the country.
Other figures from the ORR showed that East Midlands Trains received the highest proportion of complaints about carriages having sufficient room for all passengers to sit or stand (12.9 per cent).
Southern had the highest proportion of complaints about the attitude and helpfulness of staff at stations (9.9 per cent).
Meanwhile, Virgin Trains West Coast has been named as the operator with the highest rate of complaints for the 11th quarter in a row.
The operator received 196 complaints per 100,000 passenger journeys between April and June.
This happens to be the same rail line on which Virgin has just launched automatic compensation for passengers whose journey is delayed by at least half an hour.