Also Germany has a limit of 252, IIRC, and I expect the majority of long freight trains passing through Switzerland to be international services. There are plenty of old axle counters in operation around Europe because ultimately it's a limitation more in theory than in practice.
Along with the steep gradients, you have to remember there's far more passenger services on the lines, so pathing constraints force freight services to be shorter (you can't have them accelerating that much slower than passenger services, or they start taking up a disproportionate amount of capacity on the line).
Yes. Plenty of European countries have similar limits, around 250. There's plenty of old 8-bit microcontroller based axle counter systems still in use which are unlikely to get replaced until they are life expired simply because the 256 limit incredibly rarely affects anything; for reference, the most axles per train in the UK I'm aware of is 192 axles.
Switzerland runs small freight trains by US standards. Freight cars are usually 2-axle.
Here's Union Pacific's longest container train. 295 freight cars, 9 locomotives. Four axles per car. So that's over 1180 axles. A more typical US train is 100 cars and a few locomotives; over 400 axles isn't uncommon. 256 would be an inadequate axle limit in the US.
(There are longer trains in Australia, but they're usually coal or mineral hauls on dedicated track in flat country. This was a run from Los Angeles to Texas on mainline track.)
Oh, certainly, I'm not trying to claim these are the longest in any worldwide sense, but it's a common sort of maximum length around Europe.
FWIW, a lot of freight wagons around Europe are on bogies (with two axles per bogie) where they would be two-axle wagons in the US; I presume a lot of this is down to comparatively higher speeds of freight in Europe as a result of pathing around passenger services. Plenty of freight around Europe runs at up to 160km/h (~100mph), and that sort of speed is fast for a passenger service in the US. Obviously, this doubles the number of axles per wagon (though decreasing axle weight and hence track loading), further shortening the length of a 256 axle train.
Along with the steep gradients, you have to remember there's far more passenger services on the lines, so pathing constraints force freight services to be shorter (you can't have them accelerating that much slower than passenger services, or they start taking up a disproportionate amount of capacity on the line).