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| ELR | Route | Section | Remarks |
|---|---|---|---|
| ICS | Ilford Carriage Sidings | ||
| IGE | Inverkeithing North Curve | Inverkeithing East Jn to Inverkeithing North Jn | |
| ILK1 | Ilkley Branch | Apperley Jn to former Burley Jn | |
| ILK2 | Ilkley Branch | former Burley Jn to Ilkley | |
| IMG | Bamfurlong Sidings Curve (or Ince Moss Goods Lines) | Bamfurlong Sidings Jn to Ince Moss Jn | |
| INS | Inverness Harbour Branch | Inverness Harbour Branch Jn to Inverness Harbour NR boundary | |
| INW | Immingham Dock to north west entrance | ||
| IOW | Isle of Wight Railway | Ryde Pier Head to Shanklin | |
| IPD | Ipswich Docks Branch | East Suffolk Jn to Cliff Quay NR boundary | |
| ISC | Isabella Colliery Branch | Newsham North Jn to Bates Staithes | Understood to be disused |
| IUP | Ipswich Upper Yard | Understood to be part closed |