It all depends what you mean by a decent picture Peter. There's nothing wrong with yours.
If it's a reasonable light overcast sky, then the lack of harsh shadows from a direct sun can sometimes be an advantage - the clouds effectively act like a giant softbox. And shooting contra-jour also becomes less of an issue.
The challenge perhaps, on these occasions, can also come from approaching the subject in a different way. How about these as possible alternatives - I'm sure you've probably considered them anyway;
Setting a shutter speed to deliberately blur the subject to emphasise the speed?
Keeping as much sky as possible out of the composition - easier in some situations than others I accept?
Convert to black and white?
The issue I find and where I have some sympathy with you is not necessarily with the sky, but that some of the TOCs use a lot of white in their vehicle liveries. Unfortunately for you, that includes South West Trains.