Верно ли, что carriage - плацкартный вагон, а coach - купейный?

Coach - скорее с сидячими местами.

Из Британики:
coach -  Until recently the standard coach in Europe was divided into six- or eight-seat compartments, with a corridor extending along one side. These have now been largely replaced with coaches on the U.S. model, which have a centre-aisle arrangement, uncompartmented seats, and doors usually at each end of the car.

Webster - coach - a railroad passenger car intended primarily for day travel

Oxford - coach - A railway passenger or mail carriage; esp. () one not provided with beds or berths.
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Carriage - более общее понятие
A wheeled vehicle for conveying persons, as distinct from one for the transport of goods. Often in comb., as hackney-carriage, railway-carriage, travelling-carriage.

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Coach http://bbmedia.bakarbaaz.com/wp-content/uploads/2015/11/train-750x500.jpg
http://m.economictimes.com/thumb/48528580.cms?height=900&width=1200&resizemode=1
Carriage http://1.bp.blogspot.com/-z8Y0SNm5a2M/UKY9Ld_pGnI/AAAAAAAAAr8/QxPCuBnzk_k/s320/dmu2.jpg

Хотя, я так понял, carriage можно обозвать любой вагон, а вот coach - с перегородками, а остальное уже видимо от класса зависит и от расстояния.