Tom Hurry Riches, the Taff Vale Railway's Locomotive Superintendent brought out 2 similar classes, the U and the U1 for working passenger trains, both 0-6-2Ts. The only difference between the U and U1 class was in the radial wheels which were 3 ft 83⁄4 in (1.137 m) in the former and 3 ft 1 in (0.940 m) in the latter.
The boilers were identical with those fitted on the N, O and O1 classes but worked at the design pressure of 160 pounds-force per square inch (1.10 MPa), the mixed traffic engines being worked at 150 psi (1.03 MPa), or occasionally 140 psi (0.97 MPa). The main differences from the mixed traffic engines were larger diameter driving wheels, a longer wheelbase and replacing the usual four bar motion with single bar.
On 23 January 1911. locomoitive No. 193 was hauling a passenger train which was in a rear-end collision with a freight train at Hopkinstown, Glamorgan. Eleven people were killed and five were injured.