In 1949/50, the then Steel Company of Wales purchased 5 Alco S1 Bo-Bo switchers for use at its Port Talbot plant in the UK. They remained in service there (latterly with British Steel) until the early 1980's.
Four of the locos were "preserved", though whether that is still the case I haven't managed to ascertain from a scan of the internet - I think at least one of these has been scrapped - but I did see two of the survivors yesterday at the Nene Valley Railway, Peterborough.

#801 (Alco b/n 77120) is owned by the International Railway Preservation Society and is being worked on in the yard adjacent to the NVR Ferry Meadows station.
Current state is shown in the photos below, with new cab metalwork being fitted.





The owning groups website is linked below, though the dates have obviously slipped and it doesn't appear to have been updated lately.
http://www.irps-wl.org.uk/restoration-p ... jects/alco

#804 (Alco b/n 77778) is stored/dumped at the "Railworld" display site adjacent to the NVR Peterborough station. The site was supposed to be an international display of railway history, but it never really got off the ground and, apart from a nature reserve, it looks in its death throes!
The Alco here appears to have been up for sale (as mentioned on the IRPS site), but presumably with no takers. It displays a rather faded Union Pacific livery (!) at the moment.....





What the eventual fate of #804 will be is anyones guess.

The Nene Vally Railway is a bit "odd" compared with other UK preservation schemes in that it was adapted to take European loading gauge rolling stock when it was being restored in the 1970's, so there are items of rolling stock from various other parts of Europe there - particularly Scandinavia - though their numbers have been thinned over the years as the line concentrates on a "1950's British Railways" theme (just like most of all the other preserved lines here.......)

Just to update the above information, these 2 Alcos at the NVR seem to be the only two survivors out of the 5 originally delivered to the UK. One of the "preserved" ones appears to have been broken for spares some years back (the trucks are supposedly somewhere within the "Railworld" site) and the other one, owned by a group calling itself the "American Railroad Centre" on another preserved line here, cut theirs up a few years back.

In case anyone was wondering about the alterations to the cab on #801, it is to allow it to be used on the NVR, as the original one was constructed to US loading gauge - something that didn't matter within the confines of a Steel Works !

Elsewhere in the UK, we do still have an "off the shelf" US diesel loco being used on a daily basis.
Back in 1980, Foster Yeoman Ltd purchased an SW1001 from GM for use in their quarry at Merehead in Somerset. It is another one to US loading gauge, so it can't leave the quarry area. The photo below shows it back in 1982...


A preservation candidate for some time in the future???