This paper describes a major study of two rail investments in Glasgow. The main results are presented with commentary on their relevance to the evaluation of similar investments and the lessons for the decision making process that can be learned from them. During the first year of operation on the BR suburban services affected by the investments traffic was 27% above its previous level. In the inner suburbs the increase was much larger (up to 70 per cent). Traffic on the Underground also increased but the effects of changes in operation, ticketing and fares had to be isolated from the effect of the modernisation itself.