The requirements for a train tunnel are different than for a bus tunnel, which “may” be cheaper depending on what you compare to. Usually a bus tunnel will need to also accommodate diesel and gas(petrol) burning vehicles which increases ventilation requirements significantly. A train tunnel might be electric only or have diesel engines running through it. A train tunnel will need to meet the accepted standard rail vehicle envelope of the region or the isolated rapid transit system, same with road vehicles, but since the latter includes taller trucks, the height clearance would be necessarily higher than buses.
Single line width rail tunnels are used, but dual lane tunnels are the choice for some new passenger rail projects for the speed of being able to bore them even if it is more expensive.