Just keeps getting stranger.
Had a look at the two .bin files - thought it too good to be true and for once, was right. As mentioned, one is for a static loco and has no bearing on the issue at all. Spat the dummy on Saturday and uninstalled all of the non RSC DLC to see if there was something in conflict. The outcome was there was no change and now I need to reinstall it all.

RSC support haven't been able to point in any direction either.
The only variable in all of this is the home pc Vs the pc I downloaded it to. Because there is no viable net connection at home, I have RW installed on my office pc and I just update or purchase as required and then copy the steamapps folder to an external drive, take it home and copy it into the version there - to date there has been no issue. Updated as necessary, bought routes and DLC. The variable is that the office pc is running Win XP 32 and the home one is Win 7 Professional 64. The crazy thing is I fired up the office pc this morning and after an eternity managed to get RW to start up and lo and behold, the K4's are there in the asset list and can be added to a scenario, yet at home, they are not even in the asset list. The only difference is the two operating systems. This is the first DLC I have bought since I bought the new Win 7 pc for home. Is there somewhere that tells the DLC what o/s to expect? Seems far fetched, but that is the only variable between the two pc's.
It is not so much the money, but it just means the end of the hobby, because I just cant keep buying DLC on the odd chance it might work when I get it home
G