Picture rails - remove or keep?

All depends on the age of the house and would it look better to keep within the character of the property or if needed to be removed we would have to make good but shouldn't be to much depending on how it was originally fixed to the wall.

With it being a older house I would presume that they were nailed to the walls, so with some care you should be able to prize it away with out to much damage. Then fine fill with easy fill and rub down

A lot depends on how they are fitted, in some properties, they are nailed to wooden plugs, in the brick joints, which invariably come out when you try to remove the rail, which causes some plaster damage. Others are fixed in the same way, but before the finish plaster skim is applied, so leave a wide trench in the plaster, which then has to be re skimmed to get the plaster finish level. They are a period feature, that breaks up a wall surface, and can give greater flexibility in the use of colours within the room setting.

always nice to keep original features,usually fixed with nails to wood in wall,just need to take care removing,fill and paint,good to go.