Throughout its GWR and early BR operational life, the depot remained fairly intact and similar to its original layout.
The only major difference by the early 1960s was the addition of a pre-war diesel refuelling stage just north of the repair shop, for use by GWR railcars.

With a reduction in steam traction and the implementation of the Beeching cuts, in March 1964 the decision was taken to move the remaining steam locomotive allocation to the 1950s designed Southall MPD, and reconstruct Old Oak Common as a diesel depot.
Within a year the majority of the GWR 1906 depot was demolished, with only "The Factory" repair shop, furthest western turntable and parts of the stores remaining.
The main service building had 3 tracks, each holding two locos, with inspection pits, fuel supply points and a washing plant on the approach road.
Some of the inspection pits in The Factory were also lengthened and deepened and jacks provided to allow for bogie and spring changing.
It opened on 20 October 1965.
It was the last of six big diesel depots built for the Western Region, the others being Margam TMD 1960, Bristol Bath Road depot 1960, Laira Traction Maintenance Depot, Plymouth, 1962, Landore TMD, Swansea, 1963 and Cardiff Canton TMD 1964.

Just south of the residual GWR buildings, in the 1960s BR built what was initially the storage depot for the Blue Pullman trains, what later became known as the Coronation Carriage Sidings.
In the late 1970s, south of this and almost adjacent to the Great Western Main Line (GWML), BR built a depot for the new InterCity 125 fleet.
Until December 2018 these serviced and maintained the Great Western Railway InterCity 125 fleet.

In 1997 a new bespoke depot was constructed at the southern end of the site, between the Great Western depot and the main line, funded by the British Airports Authority  to service and maintain the Heathrow Express and Heathrow Connect service trains.
This was the first new privately funded train depot in the UK since the British railways nationalisation in 1948.
The inauguration of the Heathrow Express services saw the electrification of the GWML from Paddington to Hayes and onto Heathrow Airport using 25 kV overhead catenary.

On 26 July 2002, First Great Western opened a new depot on the site of the former carriage sidings to service its Class 180 fleet.

The residual GWR buildings were used from the 1970s to house and maintain singular diesel locomotives, special trains, and maintain carriages and freight stock in the area.
On the privatisation of BR, the buildings were allocated to English Welsh & Scottish, and latterly operated by its commercial subsidiary Axiom Rail.

From 2000 onwards, developments were focused  on the southern section of the former GWR site, either side of the 1970s BR High Speed Train depot.
Added to in width, this allowed this part of the depot to service all Classes operated by Great Western Railway, Heathrow Express and Heathrow Connect.
The allocation was:
Class 332 – four- and five-coach EMU used on Heathrow Express services.

Class 360 – five-coach EMU used on TFL Rail services to Heathrow
By the end of 2009, from north to south, the site layout was:
Residual 1905 GWR Factory buildings and northwestern turntable: EWS & Axiom Rail
1960's Coronation Carriage Sidings: GWR Class 180 Adelante
1970's Intercity 125: GWR 125 Intercity
1997 Depot: Heathrow Express Class 332 and Class 360
GWR Mainline
Northpole Depot - vacant
Throughout its GWR and early BR operational life, the depot remained fairly intact and similar to its original layout. The only major difference by the early 1960s was the addition of a pre-war diesel refuelling stage just north of the repair shop, for use by GWR railcars.
With a reduction in steam traction and the implementation of the Beeching cuts, in March 1964 the decision was taken to move the remaining steam locomotive allocation to the 1950s designed Southall MPD, and reconstruct Old Oak Common as a diesel depot. Within a year the majority of the GWR 1906 depot was demolished, with only "The Factory" repair shop, furthest western turntable and parts of the stores remaining. The main service building had 3 tracks, each holding two locos, with inspection pits, fuel supply points and a washing plant on the approach road. Some of the inspection pits in The Factory were also lengthened and deepened and jacks provided to allow for bogie and spring changing. It opened on 20 October 1965. It was the last of six big diesel depots built for the Western Region, the others being Margam TMD 1960, Bristol Bath Road depot 1960, Laira Traction Maintenance Depot, Plymouth, 1962, Landore TMD, Swansea, 1963 and Cardiff Canton TMD 1964.
Just south of the residual GWR buildings, in the 1960s BR built what was initially the storage depot for the Blue Pullman trains, what later became known as the Coronation Carriage Sidings. In the late 1970s, south of this and almost adjacent to the Great Western Main Line (GWML), BR built a depot for the new InterCity 125 fleet. Until December 2018 these serviced and maintained the Great Western Railway InterCity 125 fleet.
In 1997 a new bespoke depot was constructed at the southern end of the site, between the Great Western depot and the main line, funded by the British Airports Authority  to service and maintain the Heathrow Express and Heathrow Connect service trains. This was the first new privately funded train depot in the UK since the British railways nationalisation in 1948. The inauguration of the Heathrow Express services saw the electrification of the GWML from Paddington to Hayes and onto Heathrow Airport using 25 kV overhead catenary.
On 26 July 2002, First Great Western opened a new depot on the site of the former carriage sidings to service its Class 180 fleet.
The residual GWR buildings were used from the 1970s to house and maintain singular diesel locomotives, special trains, and maintain carriages and freight stock in the area. On the privatisation of BR, the buildings were allocated to English Welsh & Scottish, and latterly operated by its commercial subsidiary Axiom Rail.
From 2000 onwards, developments were focused  on the southern section of the former GWR site, either side of the 1970s BR High Speed Train depot. Added to in width, this allowed this part of the depot to service all Classes operated by Great Western Railway, Heathrow Express and Heathrow Connect. The allocation was:
Class 332 – four- and five-coach EMU used on Heathrow Express services.
Class 360 – five-coach EMU used on TFL Rail services to Heathrow
By the end of 2009, from north to south, the site layout was:
Residual 1905 GWR Factory buildings and northwestern turntable: EWS & Axiom Rail
1960's Coronation Carriage Sidings: GWR Class 180 Adelante
1970's Intercity 125: GWR 125 Intercity
1997 Depot: Heathrow Express Class 332 and Class 360
GWR Mainline
Northpole Depot - vacant
Old Oak Common TMD