How many times have EMT told me meridians can't go to Matlock??? (The answer is many more than 1).
There are two possible reasons:-
1) In the 1990s restrictions were placed on locos and vehicles (I think, from memory above RA1 but I might be wrong) using the Matlock branch due to the condition of bridges, some of which were in a poor state. The RA of the Matlock branch was reduced to RA1 (I think). I don't know what the RA of the Meridian powered vehicles are, but its probably above RA1 and thus, as a general rule, not allowed passed Ambergate Jcn.
2) Dispensation can be given (and has been several times during that period) for vehicles above RA1, usually with an associated speed restriction. The reasoning behind this is simply that it is a 1-off. The impact of one train is minimal, but one train an hour daily has a significant impact and will cause quicker deterioration of the affected structures. This is probably why they have been allowed up there today.
Of course, recent work may have been done on the affected structures and the restriction has been removed.
For information RA= Route Availability - all vehicles (including locomotives and power cars) are classified between RA1 and RA10 according to their characteristics but mainly its has to do with weight. DMU and EMU vehicles are usually RA1 whilst, for instance loaded 102T oil tanks are RA10 but empty are less than this.
Thus a journey from say Derby - Matlock may be say RA8 from Derby to Ambergate Jcn but only RA1 from Ambergate to Matlock. Thus a Derby - Matlock train must be RA1.
Its much more important for freight flows and gets quite complex where trains conveying vehicles in excess of the RA for part of the route are given dispensation with a speed restriction. The driver will have this information and its part of his road knowledge to know where to apply this restriction.
Be clear that RA does not have anything to do with route clearance for particular vehicles. Route clearance concerns itself with lineside structures, RA has to do principally with the weight of the vehicle.