I'm not sure if this is the case with Citymapper. Transit does offer alternatives, but I don't think I've ever encountered this with Citymapper, i.e., you're committed to a particular route once you start. With Transit, I often get a suggestion (while on the train) that I might not make it in time for my connection with X train, and the next Y train leaves in Z minutes. I feel with Citymapper I only get something like this if alternate trains were part of the initial route segment, eg, N/R/Q trains. I also feel Citymapper does a much worse job updating ETAs or arrival times for alternative trains than Transit.