Was away from the hobby from about 1995-2005.When I started using the railways again,the old loco's had all but disappeared replaced by the Class 66s.
The replacement of diesel vans with electric ones in my work got my thinking how did the railways deal with the 'newcomers'?
Did they (for example) give a depot 40 class66s and haul away 60 of their vintage locos to either scrap them or displace the poorer locos in other depots?
Or did they simply slowly build up the new fleet at depots, hauling away the clapped out loco's at each depot.
Quite funny at my work as those expecting a brand new van instead end up with a half done in diesel van from a depot which has went 100% electric overnight!