MENTOR currently runs in the PLPR1 test train formation. The usual formation is 975091 'MENTOR', 72631 'PLPR1', 977983 'RSC3' and a brake force runner. The brake runner can be a Mk2 BSO (usually one of 9481, 9516 or 9523), a Mk1 BG generator van, one of the Colas/NR branded ex-First Great Western motorail vans, or in exceptional circumstances a Mk2 DBSO. On occasion the brake runner might be omitted, but this is a rare occurence because it means the train has to run at reduced speed due to the reduced brake force. The PLPR1 train would usually be top-and-tailed even if a DBSO was in the formation. Usual haulage is 37s, Colas (or rarely hire-in DB) 67s and Network Rail/LORAM or GBRf class 73/9s. For a brief period (2015-16) the PLPR trains occasionally ran with top-and-tail DRS class 68s. (PLPR - Plain Line Pattern Recognition; RSC - Radio Survey Coach).