Class 31 were the most numerous of modern diesels used on the line.
 Class 31 (31101) coming off the Traws Branch at Blaenau Ffestiniog. Image by The Railway Man. |  31163 and another unidentified Class 31 at Betws-y-Coed, image by Dave Sallery. |  31134 on a nuclear train, Image by Ken Robinson. |
 31199 on the nuclear train, image by Janet Cottrell. |  31224 and 31134 on the nuclear flask train, Image by Dave Sallery |  31312 at Trawsfynydd Nuclear Sidings, image by Minfford Ken. |
 3122? and 31134 at Maentwrog Road, image by Ken Robinson |  Last official Nuclear Flask train from Trawsfynydd, a later train ran (see class 37 page) after nuclear rods were found in one of the holding ponds in Trawsfynydd. Image by Minfford Ken. |  Last official Nuclear Train, Image by Minfford Ken. |
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| Locomotive Number | Build Date (Batch) | Location Built | Notes |
| 31101 | |||
| 31116 | Operated test train with 999503 on 28/04/1991 | ||
| 31134 | |||
| 31144 | Operated test train with DM45029 on 20/09/1990 | ||
| 31163 | |||
| 31190 | Trawsfynydd Trekker 27/08/1994 | ||
| 31199 | |||
| 31203 | |||
| 31207 | Trawsfynydd Trekker 10/09/1994 | ||
| 31224 | |||
| 31238 | Operated test train with 889025, 82101, 977239 on 14/10/1993 and 16/10/1993 Trawsfynydd Trekker 10/09/1994 | ||
| 31242 | |||
| 31255 | |||
| 31258 | Operated test train with DM45026 on 25/08/1993 | ||
| 31270 | |||
| 31272 | Operated test train with 889025, 82101, 977239 on 14/10/1993 and 16/10/1993 | ||
| 31275 | |||
| 31301 | Operated test train with 999503 on 14/10/1992 | ||
| 31304 | |||
| 31306 | Operated test train with DM45029 on 08/05/1991 | ||
| 31312 | |||
| 31319 | |||
| 31327 | Trawsfynydd Trekker 27/08/1994 | ||
| 31377 | |||
| 31424 | Operated test train with DM45029 on 04/04/1990 | ||
| 31456 |