As Markus explained, some single railcars carry tail lights, usually a
single white headlight on the right-hand side, whether or not a train has a
tail. I have examples of both on file.
I can report that on the Bernina at least, observations at
the Brusio Spiral currently still show tail lamps on all trains after dark,
normally 'static' on ABe but flashing when portable lamps are added to any
tail traffic; normally freight vehicles but sometimes also empty coaching
stock being taken to or from Poschiavo from Tirano.
As I understand it, there remains a requirement to show a
red light to the rear (or at least a single white one when single headlights
are used as tail-lights at the back of solo or paired ABe), anywhere they
are running on roads with other traffic. In particular, it appears from
photographic evidence that where empty flat wagons are being towed, going by
my photographic records, a flashing red tail light appears to be mandatory.
This, presumably, is to stop traffic following an ABe down the street
colliding with the back of a low or flat wagon hidden in the dark by a flow
of car and truck headlights travelling in the opposite direction.
I can't comment on the Arosa line where that uses public
roads but presumably the same laws would apply there too. I have long
assumed (always a risky thing to do, of course, but when enquiries fail to
elicit facts the only option), that the reason 'still-green and original
cabbed' Ge 4/4i #610 (and one other I can't remember the number of
off-hand), were given a pair of small additional red lights at each end was
so they could be used in push-pull formations on the Arosa and still show
red to the rear on the road sections. Afaik, all the rebuilt Ge 4/4i were
fitted with these on rebuilding; but I may stand to be 'corrected' on that
score - as I so often am !