Sounds like Swiss trains need a single axle connector to ensure all trains have an odd number of axles.
(Did a bit of research, at least one has existed: http://cs.trains.com/trn/f/111/t/72627.aspx)
In the steam era, things were rather different. Steam power requires a linkage of rather large drive wheels, which generally means you need unpowered leading and trailing wheelsets for stability and performance purposes. The trailing wheels are almost purely to support the weight on the back end of the locomotive, but leading wheels also help with curves in addition to supporting weight on the front end. Single leading axles were quite common in the steam era, with 2-8-0 being perhaps the single most common arrangement.