I am sure when I briefly played Simutrans 64 standard, about 3 years ago road vehicles could overtake.
But in Ex. Starting in 1750 I have a Hackney carriage (twice the speed) following a pack horse over a long stretch of straight road - yet it does not overtake it. There is no other vehicle coming towards it.
Was overtaking not implemented in Extended?
Overtaking is indeed implemented in Extended, but, as with Standard, there are some quite stringent constraints: for example, overtaking is only possible if the entire overtaking manoeuvre can be completed without either vehicle passing a junction, so slow vehicles overtaking one another inside towns is unlikely. Likewise, any oncoming traffic (whether player vehicles or private cars/carts) will prevent overtaking until they have passed.