I created a multimodal network with rail, bus, and pedestrian network, and generated a 15 min service area (blue in the attached figure). I also created a single mode network with only pedestrian network, and got another 15 min service area (yellow). When I compared them, I found some area that used to be covered by 15-min walk has not been covered by the multimodal system, such as the north branch of the yellow. The issue gets really problematic as the gap increases when 30 mins and 45 mins are calculated.
Another problem is that the shape of multimodal 15-min service area is very segmented and does not make sense.
My network is clean that all road/bus route/rail route are made of links and I selected honor as the connectivity policy. No traffic restrictions applied yet at this moment.