Recent Swansea-Paddington train fires

What on earth is going on with Swansea to Paddington trains these days? In the last nine days, two trains on this service, have caught fire. More specifically, each fire has taken place in the trains' exhaust systems. I know very little about trains, so maybe the exhaust system is a common cause of train fires, or fires on certain types of train. But I find it all a bit fishy.

The first train fire happened just outside Skewen, on January 25th. Tonight, fire crews are battling a second train fire just outside Didcot. Both fires have occurred on Paddington-bound trains from Swansea, and in the exact same part of the train, the exhaust system. I cannot help but wonder if the penny-pinching train operators are failing to maintain the trains properly, or if they're using ancient and/or defective rolling stock. Or worse still, is someone sabotaging them? I think I'll be avoiding train travel for a while.