The SWT 158/159 fleet are the most reliable DMU fleet in the country, with MPC figures to make some EMU fleet managers green with envy. And this for a unit type which generally is only middling in the reliability league table amongst other operators.
What this means is that a higher level of utilisation is possible, and fewer spare units should be needed. Perhaps the large number of spares allocated to SWT helps with their reliability record- fewer sub-standard units forced out on diagrams, and more preventative maintenance carried out?
Regardless, there does seem to be plenty of evidence that SWT could live without a handful of 158s, particularly if an end was put to running DMUs on services which ought to be EMU operated. The 460s, via cascading 450s around the SWT network, would offer the opportunity to achieve this.
One of the reasons those MPC figures are achieved is that all bar two of the units make it back to their home depot of Salisbury every night. Furthermore, as virtually all units work through Salisbury at some point during the day it means that if a fault occurs during the day it is a lot easier to do a unit swap and get the unit into the depot for repair.
It shouldn't be forgotten that some of those diesel diagrams over the juice in Hampshire are also to help Fratton crews keep up their traction knowledge and Salisbury crews their route knowledge (diversionary route of Basingstoke-Southampton to then get to Salisbury via Romsey). Some of the diagrams they are used on also don't require a 4-car 450 unit, which is better used on peak services to/from Waterloo.
Finally, if it wasn't for the economic downturn you'd be seeing more of those "spare" 158s strengthening West of England services between Salisbury and Waterloo, as a couple do already. That could either mean another 10-car service (freeing up a 3-car from a previous 9-car) or adding a 2-car to existing 6-car services (like they did withe the 16:20 out of Waterloo).
So it comes down to these units being temporarily spare - it won't last for the franchise. If a couple of units were sent elsewhere that would mean overcrowding on West of England services - some still are and could do with being strengthened as it is.
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The route to Exeter and Salisbury would seem to be one where LHCS or even Adelantes could be used freeing up quality DMU's for use elsewhere. The DMU shortage is going to get worse and we seem to be making the problem worse by some crazy allocations.
Really? The benefit of the 158/159 fleet is that it gives SWT huge flexibility in train formations on the West of England services. Currently through the day they operate a mix of 3-car, 5-car, 6-car, 8-car, 9-car and 10-car services. The Exeter services generally detach/attach a unit (or units) at Salisbury so that they are not carrying lots of air down West; and/or have a Bristol portion attached too on a few services.
There are only 5 spare Adelantes now East Coast don't want them - hardly a flexible fleet choice for SWT to operate alongside the 158/159 fleet. In addition, what spare LHCS stock is there for SWT to operate on the West of England route? Who would maintain it? Who'd pay to train the Drivers/Guards on it? Is Waterloo cut out for regular locomotive operation these days - Using a 67 and a DVT means you could only have 8 23m coaches in a platform - a loss of capacity on 9-car and 10-car DMUs.
Arriva have 5 150/2 units on lease to FGW that are due to come back to ATW soon. Should they be sent to Northern instead because Arriva obviously has too many spare units if it can afford to lease them out?
) And people couldn't complain about a lack of airport stations; they would serve Brum and Gatwick, and could always be diverted - if the route is electrified; if not, get to it! - via Manchester as well. Any turbo chargers available? 
