Firstly the details I've collected are not gospel but do carry some confidence in what can be suggested. If you can clarify any better happily comment below.
This all started with some peak hour and Saturday services were made of one Class 37/4 locomotive and four Mk. 2F carriages. Loco 37425 and 408 was involved in collision at Rhymney Sidings on 30 July 2005, thus 408 was withdrawn, and replaced by 419.
When 425 went off to be repaired Class 47 loco's were hired from Riviera Trains to cover.
The Class 37-hauled trains ended on 10 Dec 2005, so loco 37411 went to be repainted into 1960s BR Green livery and 80s BR Blue Large Logo livery for 37425 to mark the end of these locomotive-hauled trains.
DMU's were introduced to the line at this point but Arriva had many complaints from passengers about over crowding. So 37410 returned to run in peak times in the week to alleviate this issue. So you see quite a bit of history with class 37 trains on this line.
In December 2006 it was finally agreed and announced the loco hauled service would cease and that would be the end to the 37's on this line. This has been the case including into the transformation of the rail services into Government hands from 14th October 2018 as Transport for Whales (TfW).
However now it's said the class 37's will be back on this section soon. Negotiations were made between TfW and Colas to hire two of the three class 37 locos and two sets of four coaches to run on this line.
Leaving one loco spare the current soon to be surplus 4 class 150 sets will be refurbished then put in service on the Chester to Liverpool diagrams meeting January's 2020 accessibility deadline. This will also make the exit of any class 142 and 143 sets much quicker.
Colas class 37418 has been spotted near Cardiff Canton, thus be used and it's said it's already on the line ready for drivers to train and refresh in them. 37421 is supposed to be out of service at the moment but will join the threesome soon.
According to TfW DRS will provide the third loco 37175 and keen to introduce this service when possible. Sources suggest the 20th of May is the most probable date.
37175 is reported to be en-route via Nottingham Eastcrofft for a rescheduled bogie swap with 421 that will be sent over after to provide a service.
So to recap it does seem three class 37's - 418, 421 and 175 will provide 4 services at day from Cardiff Central along the valley line via Heath and Llanishen in the north of the city, to Caerphilly, Bargoed and Rhymney Railway Station.