According to Swedish KTH railway group, in long trains surface friction from sides and roofs accounts for 27,0% of air resistance, the underfloor equipment for an additional 7,5 %. Since these effects are roughly proportional to train length (which is not true for effects from head and tail!), they play a smaller (but still significant) role in short trains.

There is some potential for improvement by

• streamlining the outer shell
• finding surface coatings for train sides and roofs that minimise surface friction
• optimising coach transition areas (which cause major flow separations). ETC recommends that gaps between consecutive coaches should not exceed 250 mm.
• optimising arrangement of windows, doors and door steps
• underskirting, i.e. covering the rugged structures of the underfloor surface by a smooth cover.