The only railroad “low back” I am aware of is the fireman's “jump seat” (aka lumbar seat.) Most commerical airlines still use them as well. What, pray tell, is a “low back(ed)” rail car?

Fwiw: A low-back(ed) car(t) was a kind of unsprung, horse-drawn, two-wheeled flat-bed and a very popular Anglo-American song & stage reference:

“...commonly used here, and I believe on account of the roads, but it is a disagreeable species of conveyance; you cannot help forming to yourself, from the slowness, its humble appearance, and your near acquaintance with your mother-earth, while you are riding in one of these carts over the sandy roads, that you are in a sledge, drawn like a traitor, to some place of execution; but when upon the rough pavement in the town, you are electrified with the bone-breaking motion, almost impossible to sustain it even when in health, but an invalid is in danger of annihilation.”

[Carey, G.S., The Balnea or, An Impartial Description of all the Popular Watering Places in England, (London: J.W. Meyers, 1799, p.261)]

Typical: Lyr Req: The Low-Backed Car (Samuel Lover)