just a SIMPLE Example:
Trucks are meant to carry HEAVY-BULKY Goods , so for the truck owners , the only thing matters is Safe and NORMAL SPEED , thats the reason why they get Money as per AMOUNT of load they carry.
BUT
Buses are meant to carry people and are bound with a "TIME CONSTRAINT". They cannot waste time , so they are...
more...

made for GOOD SPEED. Thats the reason why They get paid in terms of Daily Rounds they made, not on the Quantity in ONE GO , but combined passenger quantity they carry in a day.
So the Buses are thats why made Lighter and Streamlined for achieving 100 kmph speed quickly as easily. But the trucks are made Bulky and weightier so that due to heavy load they don't get exhausted and still could pull the Heavy Load. Thus their body made for Low Speed but Greater Capacity.
Same is with Goods train and express trains. Goods train and Their Locos are meant for HEAVY DUTY LOAD CARRYING whereas Express trains & their Locos are meant for HIGH SPEED. Curving or Banking is a completely Different phenomena which is NOT at all related to Train's length.