just like PhilP and dunnyrail mentioned - metallic rails to hang up curtains.
here in paraguay i could buy cheap tin rails, that were either bronze- or tinplated. - and cheap! (about $0.20 per yard of rail)
a friend with carpentry tools made me the ties from offcuts. (cheap too)
main difference between out- and indoors was the glue used. outdoors i used the german UHU, or the "3M" goop used for motor sealing, indoors just white wood-glue.
(hindsight: for outdoors, now i would drive a thin nail through the tin-rail into the middle part of the tie every tenth tie, or so)
btw: if there are no more metallic curtain rails... for battery driven locos the plastc rails might be even better. no rust, paintable. just using a tacker, to connect rail and "middle ties".
just that. the middle piece of the "tie" for the correct spacing. the outer pieces were just for optics.
cause in the old days the next shop to develop negatives, was 300miles/500km away, i got only some pics from the later indoor layout:
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