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| ELR | Route | Section | Remarks |
|---|---|---|---|
| PAA1 | Pirbright Jn to Alton | Pirbright Jn to former Farnham Jn | |
| PAA2 | Pirbright Jn to Alton | former Farnham Jn to Alton | |
| PAC | Patchway Curve | Patchway No. 1 Jn to Filton West No. 1 Jn | |
| PAR | Par Loop | Par Loop Jn to St Blazey Jn | |
| PAS | Portsmouth Blackfriars Jn to Portsmouth and Southsea Low Level | ||
| PAT | Caterham Branch | Purley North Jn to Caterham | |
| PBC | Pinxton to Bentinck Colliery Line | Langton Colliery Branch Jn to Bentinck Colliery | Understood to be disused |
| PBJ | Proof House Jn to Bushbury Jn (via Bescot) | ||
| PBL | Perry Barr North Curve (or Loop) | Perry Barr North Jn to Perry Barr West Jn | |
| PBN | Preston Fylde Jn to Blackpool North | ||
| PBS1 | Pye Bridge to Shirebrook Line | Pye Bridge Jn to Kirkby Summit Jn | |
| PBS2 | Pye Bridge to Shirebrook Line | Kirkby Summit Jn to Mansfield | |
| PBS3 | Pye Bridge to Shirebrook Line | Mansfield to Regional Boundary | |
| PBY | Portbury Docks Branch | Portbury Jn to Portbury Docks | |
| PDB | Preston Fylde Jn to Deepdale | Understood to be disused | |
| PDL | Pelaw Metro Jn to NR Boundary | Down Link Line | |
| PED1 | Penistone to Doncaster | Penistone Jn to former West Silkstone Jn | |
| PED2 | Penistone to Doncaster | former West Silkstone Jn to Quarry Jn | |
| PED4 | Penistone to Doncaster | Wath 13M 33ch to former Mexborough East Jn | |
| PED5 | Penistone to Doncaster | former Mexborough East Jn to Doncaster South Yorks Jn | |
| PEF | Ferrybridge Curve | Ferrybridge South Jn to Pontefract East Goods Jn | |
| PEH | Penistone and Huddersfield Line | Springwood Jn to former Huddersfield Jn | |
| PEM | Pembroke and Tenby Line | Whitland Jn to Pembroke Dock | |
| PHC | Priory Yard to Hull Central Goods Line | Hessle East Jn to Dairycoates | |
| PHS | Peterborough Holding Sidings | ||
| PJL | Parkside West (or Lowton) Curve | Parkside Jn to Lowton Jn | |
| PJW | Portobello Loop (Wolverhampton) | Portobello Jn to Heath Town Jn to Crane Street Jn | |
| PLG1 | Park Lane Jn to Greensfield Jn | Park Lane Jn to former High Street Jn | |
| PLG2 | Park Lane Jn to Greensfield Jn | former High Street Jn to Greensfield Jn | |
| PLO | Plymouth Loop No. 1 | Lipson Jn to Speedway Jn | |
| PLS | Plymstock Branch | Understood to be part closed | |
| PMJ | Manton Jn to Peterborough | ||
| PMT | Polmont Jn to Larbert Jn | ||
| PNS | Cowlairs Chord | Cowlairs South Jn to Sighthill West Jn | |
| POC1 | Port Clarence Branch | Billingham Jn to Billingham Jn to Phillips Petroleum Depot Jn | |
| POC2 | Port Clarence Branch | Phillips Petroleum Depot Jn to Port Clarence Branch | |
| POD | Portishead Branch | Parson Street Jn to Portbury Jn | |
| POR | Porthcawl Branch | former Cefn Jn to Tondu Jn | |
| PPA1 | Phillips Park and Ashburys Branch | Phillips Park No. 1 Jn to Ardwick Jn | Understood to be part closed |
| PPA2 | Phillips Park and Ashburys Branch | Ashburys West Jn to Old Midland Jn | |
| PPA3 | Phillips Park and Ashburys Branch | Ashburys West Jn to Ashburys West Jn | |
| PPH | Preston Park (or Cliftonville) Spur | Preston Park Jn to Hove | |
| PPP1 | Philip’s Park Jn and Baguley Fold Jn Curve | Park Station Jn to Baguley Fold Jn | |
| PPP2 | Philip’s Park Jn and Baguley Fold Jn Curve | Park Station Jn to Philip’s Park No. 2 Jn | |
| PPW | East Putney Branch | Point Pleasant Jn to Wimbledon TfL Platforms | District Line |
| PRA | Aylesbury Branch | Princes Risborough to Aylesbury South | |
| PRC | Watlington Branch | Princes Risborough to Chinnor | |
| PRG | Rose Grove West Jn to Padiham Siding | Understood to be disused | |
| PRL | Park Royal Goods Branch | Understood to be disused | |
| PSE | Robin Hood Line | former Regional Boundary to Shirebrook East Jn | |
| PSF | Perry Street Fork Jn to Slade Green Jn | ||
| PSP | Parkgate Tram Transfer Line | ||
| PSR1 | Preston Strand Road Branch | Preston Ribble Jn to COM | |
| PSR2 | Preston Strand Road Branch | COM to Preston Docks NR boundary | |
| PTA | Penalltau Branch | Ystrad Mynach South Jn to former Penalltau Jn | |
| PTD | Port Talbot Docks Branch | Understood to be disused | |
| PTH | Penarth Branch | Cogan Jn to Penarth | |
| PTP | Port Talbot Central to Pontyrhyll Jn | Understood to be disused | |
| PUL | Pelaw Up Link Line | Pelaw Metro Jn to NR / Tyne & Wear Metro Boundary | |
| PVS | Nuneaton South Jn to Park Viaduct South | ||
| PWE | Pontardawe Branch | Felin Fran Jn to Clydach | Understood to be disused |
| PWS1 | Paddock Wood to Strood (via Maidstone West) | Paddock Wood to Maidstone West | |
| PWS2 | Paddock Wood to Strood (via Maidstone West) | Strood Jn to Maidstone West | |
| PYE1 | Pyewipe Branch | Immingham East Dock Jn to ELR change | |
| PYE2 | Pyewipe Branch | ELR change to Grimsby Docks NR / ABP boundary | ABP: Associated British Ports |
| PYE3 | Pyewipe Branch | Grimsby Docks within ABP boundary | ABP: Associated British Ports |