Just to clarify this is the BR Class 40 diesel locomotive:-
http://en.wikipedia.org/wiki/BR_Class_40AP have done a superb job of recreating these iconic diesels for the sim. Excellent external model and cab with many scripted features, you don't just put the driver's key in and go. The centrepiece is (naturally) the magnificent AP sound set, newly recorded from a preserved example of the Class. I travelled behind a number of Class 40's in the late 1970's and early 1980's and the audio mix captures well all the little nuances including the famous exhaust whistle, the turbocharger and even field diverts kicking in. Just one caveat, as with other AP add-ons, the sound heard in the cab is authentic, that is to say you don't really hear the "thrash" even with the door and windows open. You have to use head out or helicopter view if you want to hear the full effects. I know this might be realistic but it's a bit of a shame there isn't an option to exercise some licence and have the exterior sounds audible in the cab. After all, that's why we run these big diesel locos! Track sound is excellent and about as far as you can get from default xylophone jangle.
As to the physics, well I can safely say this is one loco that won't elicit comments about powering uphill in Notch 2 with a 500 ton train. The real 40's were fairly ponderous beasts and that seems well reflected here. I wasn't quite so impressed with the braking curve though, seemed to be alot of deceleration for a very slight application (whether air or vacuum). Heavy trains from this era would be looking at around a mile to stop on a full service brake application, from 90 MPH.
The pack includes the former Digital Traction BR Mark One coaches and there is a free supplementary stock pack which includes items required by the scenarios. Unfortunately all the scenarios I tried to run were looking for further payware items over and above what was included or in the free pack. I know this is a debateable point but I do feel if you're selling a product then the lion's share of scenarios should run with what you get. Having just paid £19.99 I want to thrash the beast, not have to start swapping out consists with RW Tools, faffing about in the scenario editor or making do with ersatz Quick Drive.
Overall 4.5/5 and an essential purchase if you like classic diesels or motive power which requires a bit of thought to drive.