Wouldn‘t those contribute to wheel slip?
simple, you can get a coupling form galvenized to copper, in the uk,, the manufacturers that make them are johnsons, dont know anywhere else, im sure you can get them in other conuntries. these are use on pipes without threads.
Use a piece of plastic pipe rated for potable water
Idler axles are necessary on some locomotives to spread the weight. They were especially common in the earlier days of diesel-electric traction because of the heavier weight of the equipment, compared with today. For example, here in the UK, the 2000-2300 hp locomotives built in the mid-1950s had the 1Co-Co1 wheel arrangement, i.e., two eight wheel bogies (trucks) with three axles with traction motors, and the leading one idling. In other words, they only needed a total of six traction motors, but this would have meant that the maximum axle-loading would be too high for the track. Within five years improvements in the design of the engines, generators, control gear and motors meant that locomotives with greater power had the same six powered axles, and no carrying axles, i.e. the Co-Co wheel arrangement. You can therefore appreciate that the only way that idler axles could contribute to wheel slip is if they were unnecessary in the first place!
Another case for an idler is when a traction motor has gone bad and come apart inside, so that the axle cannot turn. An idler axle with no gears on it, will be brought out so the locomotive can be taken to the shop for repairs.
Chiel has part of the answer. Another reason (typically) is to provide stability at high speeds, such as the EMD E-series passenger locomotives; their trucks' center axle was just an idler, not only for stability, but also because the truck bolster served as a conduit for the cooling air blown into the traction motors. BNSF rosters a fleet of ES44AC4s, whose center axle is unpowered. When the train gets above a certain speed, that center axle automatically raises, to concentrate the weight on the powered axles.
