Locomotive 37901 ‘Mirrlees Pioneer’ on the Severn Valley Railway during the Class 37 Gala Originally built as D6850 (later renumbered 37150), this locomotive is part of the long-serving Class 37 fleet constructed between 1960 and 1965. Out of 309 built, only four were later converted in 1986 to trial new technology for a proposed Class 38 — becoming 37901 to 37904. What makes 37901 truly unique is its Mirrlees Blackstone MB275T engine, fitted as part of that experimental programme. While the Cla