I believe Network Rail has several measurement trains. The premier one is the "New Measurement Train" which was converted from a HST▸ and has two class 43 power cars, and can run at 125 mph. Another - the one in your photographs - is four coaches top-and-tailed by ordinary locomotives such as class 37s, and has a lower top speed; and there may be some other small ones.
There are more types of measurement train - but (since this is the railway) the more detail you dig up the less sense it makes.
Here's a description of several. It's a couple of years old, and there may have been some changes since.
That 1Q18 (there are others!) ran under TSC 52495117. This is one of a set of Colas infrastructure measurement train codes:
Colas Infrastructure 52495100 NDS Infrastructure
Colas Infrastructure 52495110 COLAS NSC T.R.U
Colas Infrastructure 52495111 COLAS NSC T.R.C
Colas Infrastructure 52495112 COLAS NSC U.T.U
Colas Infrastructure 52495113 COLAS NSC P.L.P.R
Colas Infrastructure 52495117 COLAS N.M.T
Then if you look in the national bit of the
TPR▸ , as well of loads of explanation about timetabling, it has a schedule of these trains. It does not give the TSC, and the names given to the trains don't match that list: these are LD75, UTU-R, UTU-T, HSMT, HST7-125, and (just once) HST8-125. This column is headed "model train", so it ought to be the timing model (i.e. timing load) alone. But I can't get it to match the
WTT▸ entry, which shows timing load with the usual types, any more than it does the measurement train type.
Also, the runs are shown per day and some are split into a number of train IDs ... but the model train listed can change between them, despite this being the run of a single train. And timing load can change within the same Train ID anyway - the 1Q18 example above starts as "HST", then becomes "Diesel locomotive, trailing load 245 tonnes at 90mph", then "HST" again, and finally "Diesel locomotive at 75mph".
