I found some videos recently of 'street runner' trains and thought it's a shame tracks can only cross roads in OTTD. This could be useful for expansion of infrastructure inside developed cities without annoying the local authority. I appreciate implementation would likely be very difficult however, and with the multi-car cargo trams available in some GRFs (eg. GRVTS and HEQS) not really necessary.
Here's a demonstration: https://www.youtube.com/watch?v=9iG0xOAgWMI&t=3m
Road vehicles can typically run parallel to the train on either side, with the track in the centre.