37884 and i hear 37800, which is currently at Leicester, are to be mainline reinstated and fitted with dellner couplings for hauling class 321's from Ilford north to Wabtec in Doncaster for refurbishment. The couplings won't be the same as the class 57/3 versions as these type will i think be coupled onto the buffers at one end so they can haul the unit set without a barrier coach / wagon.
As has already been mentioned it's the Rail Operations Group who have leased the 2 locos off Europhoenix. The ROG have won a few stock movement contracts in the last few months. They also work the moves for Southeastern class 375's to and from Litchurch Lane, Derby. They've recently won the GWR contract to move mk3 stock to and from Kilmarnock off DBS. They also move other units / DMU's such as 150's & 158's for LM and ATW but not hauled by locos.
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As for any unused class 37's there aren't any left in DBS hands but there are a few others either under the control of DRS, HNRC, WCR, NR or preserved that could be repaired and reinstated i would guess.
DRS - 37503/510/521/670/714 (Barrow Hill), 703 (Bo'ness & Kinneil railway), 37038/194/682 (Derby spares donors), 716 (Derby repaired)
WCR - 37165/214/517/710 (spares donors)
NR - 37198 (currently at Nemesis rail repairs ?)
UKR - 37905/906 (Leicester TMD)
SRPS - 37025/413 (repairs to mainline standard Bo'ness & Kinneil railway)
There are over just under 30 other examples of class 37's preserved / privately owned around the country working, under repair or stored in a poor state of repair. But that didn't stop DRS. Just look at their decision to purchase 37407 & 424 from the Churnet valley railway and 37413 from the Bo'ness & Kinneil railway and have them repaired and reinstated at RVEL Derby.
Preserved / privately owned examples include 003/009/023/029/032/042/075/097/108/109/142/152/215/227/240/248/250/255/263/264/275/294/308/314/324/418/674/679/901. Which when all said and done if any FOC made a decent offer to the owners then you could see any of these return to the mainline. Such the position of the emissions regulations at present and the allowance of purchasing old locos and having them repaired to a mainline standard.
Colas have recently purchased 37057/116/175/219/421 and returned them to the mainline. With others 146/188/207 & 254 due for repairs.