Concentrating certain classes of unit onto certain services has operational benefits above and beyond dealing with coupling incompatibility, especially for a geographically large TOC like TPE and, to a slightly lesser extent, ATW. It means that the traction knowledge required for traincrew can be concentrated onto certain depots (I expect that only a few TPE traincrew depots "sign" Cl170s) which is more efficient. It also simplifies ticketing and seat reservations because it is known that a certain type of train will always operate a certain service. And yes, it means that dealing with a total failure is easier because it is more likely that a compatible train is available to push the duffer out of the way.
However, I will repeat yet again that the number of times that a train becomes a total failure and requires assistance is tiny. Most failures that require a train to be removed from passenger service will not totally immobilise it. In addition, it doesn't matter if TPE operate a mixed fleet of incompatible trains because if a Cl185 "sits down" and requires to be assisted, the nearest train to offer that assistance will be another Cl185. Likewise if it's a Cl170 that decides it doesn't want to play any more. Therefore the mixed nature of the fleet does not have a negative impact.
So in summary, no I don't believe that there is any real advantage. I'm sure TPE would have loved to have a single fleet of one type, but the fact that they do not have this luxury is not a problem. They have allocated their fleets in such a way as to make the most efficient use of them and remove any possible issues that inter-working might create.
I am not sufficiently familiar with ATW's operations and fleet disposition to comment on the situation there, but I imagine that they have taken a broadly similar approach.
**EDIT**
One other thought I would just like to add is to address the question regarding rolling stock cascades.
The rail industry is not dumb and is already aware of the potential issues. The last few rolling stock cascades have seen units moved with some thought about where they are heading in order to ensure a like-for-like transfer. For example, LM's Cl150s went to Northern and FGW who already have their own fleets of these units. And there are others. SWT sent their Cl170s away in exchange for Cl158s to ensure some degree of commonality with their fleet of Cl159s, with the Turbos going to existing Cl170 operator ScotRail. Likewise at the formation of XC and EMT the Cl158 and Cl170 fleets were divided between the two in such a way as to ensure that two distinct fleets were created giving each operator a single class of unit.
O L Leigh
