I wonder why don't more places do this. I mean surely the actual carriage of the train can be reused, but with just the engine refitted.
(photo: https://upload.wikimedia.org/wikipedia/commons/d/d4/Coche_La... )
If repairs become too frequent, fewer trains are available for use on any given day, which means not all services can be run.
Old carriages might not meet modern expectations of crashworthiness or fire resistance.
While the bumpy ride is just a nuissance, the noise of those old junkers probably should require everyone to wear heavy hearing protection, if one had to adhere to workplace regulation.
When their crooked boogies start to screech and grind in turns, the noise is physically painful
Since we're talking about Berlin, the new airport had a similar problem: Because of all the delays and problems, the construction permit expired. And it could not just be reissued for the same plans because safety and energy conservation regulations had become tighter in the meantime. As far as I know, parliament extended the construction permit manually to avoid the replanning costs.
Vivarail (http://www.vivarail.co.uk/) are doing almost exactly this in the UK with late-70s London Underground trains, fitting modern electronics and Ford truck engines as generators for the electric motors. I think it's a solid idea.
Nevertheless, there are plenty of really old trains still running. Service times of fourty years and more are the rule, not the exception.
Is this different between trains and trams? In Dresden, our rolling stock of trams is between 10 and 20 years old (with half a dozen older trains reserved to cover high load situations), and the tram operator is already loudly thinking about ordering the next generation.