As Jason's map clearly shows, the only North British line penetrating an area with a significant Gaelic-speaking population was the West Highland line and Mallaig extension. Even there, it would appear that literacy would be in English; any inclination to speak Gaelic at school would be beaten out of a child - see what the National Library of Scotland has to say on the subject. For comparison, the only pre-Grouping railway company notices I have seen that include Welsh are of the "Trespassers wi