Lovely! Thanks all for posting these... Anything railway-related like this I love...
The Severn Valley Railway have just had a very successful Gala for the Class 50 Diesel... There are variously known as 'Hoovers' or 'Vacs' on account of the noise they make but recently they have been re-christened 'Logs' due to the amount of clag (smoke) they generate running slowly in preservation - they look as if they are burning logs
The fact is that big mainline diesels like the 50 are barely ticking over when running on speed-limited preserved railways and it shows as spectacular clagfests

Just imagine idling an XUD for several hours and then cracking open the throttle... That'll give you a good idea of it!