Practically all factories doing final assembly work have sourced their material from other sites or companies. Aircraft factories don't make all the parts on one site, large chunks are made elsewhere and then put together. Large chunks of the Queen Elizabeth aircraft carrier were not made at Rosyth, but only assembled there. Cars are frequently assembled in plants where practically everything - engines, seats, windows, trim, wiring - are made elsewhere.
There is no longer any correlation between the size of the final assembly plant and depth of manufacture. This correlation is a very 19th century concept.
Agreed - and also assembly and fitting out of trains is labour intensive because you're having to fit a lot of equipment into a small space (just think about all the wiring and plumbing behind the panels, as well as everything under the floor). So from a jobs point of view, the final assembly plant is probably one of the better parts of the production chain to have.
The electronics business (where I work) is probably a good example of today's globalised, fragmented production - a UK company selling their own products into, say, the industrial capital equipment sector, might contract with someone to design it (my end of the business) then with another UK company to manufacture it for them.
That sub-contract manufacturer then buys a variety of standardised component parts (usually via distributors) that are made in high-volume production plants all over the world. It might buy the printed circuit board (the custom made panel that the components are soldered to which wires them all together) from another UK company, then it assembles components on the board, puts it into an enclosure (which has come from yet another supplier), maybe adds some hand wiring to connect circuit board to connectors and switches etc. on the enclosure. After testing the finished product and packaging it, it ships it to the original 'product' company.
So in the scenario above, the company that is actually selling the end product actually does very little of the manufacturing (or the detailed design), but it does support a whole hinterland of jobs in the UK and around the world. This is the essence of 21st century manufacturing, at least at the lower volume end of things (and trains are not high-volume products).