I tend not to criticise grainy photos as at the time of publication, the only existing photos of a rare something or other may well have been taken in poor conditions by an enthusiast with what effectivly amounts to a Box Brownie. I'd rather have a contemporary picture than not, even if the quality is relatively poor.
As for colour, early film aimed at the amateur market was expensive and rarely used. Unless in the cinematographic industry, professionals were unlikely to use colour to record railway subjects. In addition, pre-WW2 colour film was very grainy and impressionistic in terms of colour balance, so a colour image prior to the late 50s may be helpful, but should be taken with a pinch of salt!