Cutting the main evening commuter train between Wales' biggest cities has saved First Great Western more than £1m a year , the company claims (link below.)
http://www.rmtbristol.org.uk/2007/01/the_insane_economics_of_uk_rai.html#more

The figure is not the cost of running one daily train over the 40 miles from the capital to Swansea. By not extending the 5.18pm to Swansea , FGW can operate its timetable with one fewer train. The cost of leasing a train and paying the driver and conductor is £1.2m a year.

Swansea transport consultant John Davies , who was British Rail's manager for Wales in the 1980s , said the main problem after withdrawal of the 5.18pm was overcrowding on the 5.21pm to Maesteg , which was besieged by Bridgend commuters.

Adding extra coaches was impractical because stations on the Maesteg line , opened to passengers in 1992 , were only long enough for two coaches.