It was said at the time (in Modern Railways, probably around 1990 from memory) to be a plan to get rid of the remaining first generation DMUs on Regional Railways. Obviously it didn't quite work as some of the 101s survived for another decade and we still have a couple of bubblecars today. Possibly the growth we have seen since the 1990s had started to kick in even then.
In the "Network SouthEast Story" it states that NSE was trying to get as much growth in the mid 1980s before the economic cycle took its next downturn so that whilst times were good they could get the maximum amount of investment in new trains. Both NSE and Regional Railways (RR) started to struggle towards the end of the cycle. Hence what has been mentioned about RR finding itself with too many 158s on order and NSE taking 66 vehicles for the West of England. In turn, NSE agreed to take over the Oxford-Worcester services from RR (An original 155 route I believe) because releasing 66 158 vehicles for NSE was gping to leave RR slightly short. I think that is why 165001-005, originally ordered for Chiltern Lines services, became based at Reading to help backfill for 166s covering services to Worcester.
The 153s were designed to help RR get rid of the last first gen DMUs, and were largely intended to operate as single cars. However, post-privatisation growth quickly saw them used in pairs or to strengthen other trains to three cars as the arrival of third generation DMUs saw second generation DMUs get cascaded.
Heaton Depot (HT) got 153s for use on branches in the North East and also the Cumbrian coast. The former helped with the cascade of 143s to South Wales and the South West; the latter saw Class 108 DMUs withdrawn.
Norwich Crown Point Depot (NC) got 153s for use on East Angian branch lines alongside some Class 150/2 units where a little more capacity was needed. This helped allow the withdrawal of the remaining first gen units, plus assisted with the cascade of 156s to Tyseley Depot (TS) with the 158s operating the long distance stuff. I recall that at least one Centro livered 150/2 and one Merseyrail liveried 150/2 were sent to NC (gaining RR colours) as RR "pinched" the 150/2s from several areas for NC.
Laira Depot (LA) got 153s for use largely on the Devon and Cornwall branches alongside 150/2s. They again were used to replace first gen DMUs (including the Class 122 bubble cars), although it shouldn't be forgotten that these were largely brought in from elsewhere after the first attempt to replace first gen DMUs with Class 142s (branded as "Skipper" units and in a
chocolate and cream version of the then
Provincial blue railbus colours) failed due to the use of non-bogied stock on the tightly curving branch lines.
As the economy started picking up again in the 1990s RR had to refurbish some first generation DMUs to pick up the slack. Some of these (Class 101 power-trailer formations) were originally allocated to NC until they got all their 153 and 150/2 units, and they then went to Longsight Depot (LO) for use in the North West alongside Class 101 power-twin formations and a handful of three car formations. Some Class 101 power-twins also went to Scotland for use by Strathclyde PTE intheir orange and black colours. RR also refurbished some Class 117 three-car units, initially in use from Tyseley Depot (TS) and then transferred to Haymarket Depot (HA).
