When the time came to consider replacing the well-life expired (understatement) class 50s, 47/7s and early Mk2 coaches, BR gave consideration to a number of options:

* re-engineering and further life extension of the existing stock

* construction of new diesel locomotives (the proposed Intercity diesel class 48 was under consideration) and new hauled coaching stock

* new DMU stock, either the class class 158 already ordered and under construction for Regional Railways, or the proposed class 171 turbo (an air-conditioned and within standard loading gauge version of the class 165)

* electrification from Worting Junction to Exeter, using new and/or cascaded "heritage" EMUs

* use of cascaded Intercity 125s, in either 1+5 or 2+8 formation

The Department of Transport was in favour of the IC125 option, but the stock was unsuited to the route and the inflexible fixed formations would have resulted in running far too many empty seats west of Salisbury.

A study concluded that (remarkably) the electrification and new EMU stock or new DMUs were the two viable options. However, since the cost of electrification had to bear electrical infrastructure as well as stock construction costs, whereas the DMU option only involved (mostly) rolling stock costs, DMUs it was to be. The Network SouthEast business unit was said to be in favour of the proposed class 171, however at this time the prevailing economic downturn had left Regional Railways committed to more 158s than it then had need for. The number of spare 158 cars (60 cars) were nine cars short of the Network SouthEast requirement, but following negotiations, NSE agreed to pick up Regional Railway's share of the Paddington to Worcester route (shared with Intercity) which released sufficient 158 cars from RR to NSE. To run the acquired Paddington to Worcester route, NSE added a further six cars to its order for class 166s (the 171 proposal having by then been dropped).

One benefit of NSE taking up part of the 158 order was that it obtained its new rolling stock quicker than with any other viable option. NSE drew its stock from the 3 car 400hp 158 variant. However, the interior layout was not to NSE's liking, particularly the 2+2 seating in the first class (NSE preferred 2+1) and the buffet trolley stowage arrangements. Remember - this was in the early days of BR having to work with professional procurement procedures between it and its newly privatised suppliers, and not merely issue a late change of specification to its own workshops. It did not prove possible to arrange a mutually agreeable variation order with BREL, so as a result standard specification RR 158s in NSE colours were shipped to another contractor at Rosyth Dockyard to perform the conversion to what had become new class 159.

After considering various options, which I will not go into here, to house and maintain the 159s a new depot was built at Salisbury on the site of the former GWR station. In service the 3 car 159s work in multiple, dropping off excess capacity at Salisbury as necessary according to the timetable.

The 159 units were built at BREL Derby and are of welded aluminium construction. They consist of :

* DMCL diagram DP322 lot number 31051 weighing in at 38.5 tonnes seating 24 first (facing) and 28 standard (facing and unidirectional) with one toilet suitable for the disabled

* MSL diagram DP209 lot number 31050 weighing in at 38 tonnes seating 72 standard (facing and unidirectional) with two toilets

* DMSL diagram DP260 lot number 31052 weighing in at 37.8 tonnes seating 72 standard (facing and unidirectional) with one toilet and a parcels area.

The cars are 23.21 long by 2.82 metres wide, and each is powered by one Cummins 14 litre NTA855R 300kw (400hp) engine feeding a Voith T211r hydraulic gearbox and torque converter , which then feeds a Gmeinder final drive. Each car has one BREL P4 and one BREL T4 bogie. Maximum speed is 90mph. The units are gangwayed throughout and the platform side doors are of the sliding plug type.