5199 at Glyndyfrdwy in December 2018. © Andrew
The GWR Class 5101 (aka ‘Large Prairies’) were a class of medium sized tank engines produced for the GWR and designed by Charles Collett. Originally built to haul suburban and local passenger services all over the GWR network, the class later found use on support duties such as banking and piloting as traffic declined following World War II. Several members of the class have been preserved and can be seen in use today.
(Information provided via Wikipedia)