I'd suspect that it is partly historical accident where top contact was tried first and spread because they'd got it working (kind of like standard gauge, more arbitrary than engineering optimisation).
Obviously this is why there's no real adoption, but CRE systems tend to be an idiosyncratic bunch, and, especially for something so oft-quoted as a fully-fledged type of CRE, you'd think there'd be at least be a few systems built over time.
You may get more issues with gauging. Top contact shoes spring down into the space where a third rail would be anyway. To spring against a side contact rail, the side contact shoes probably rest wider on both sides, and may need some electrical clearance on top of physical clearance depending on the details.
I suppose if this could ever be an issue it'd be an issue on the British loading gauge, but given a difference of maybe inches at most, I wouldn't see it being too much of an problem in the outside world.
Although I can see the safety advantages to those working on the line, bear in mind usually if one shoe is live all shoes are live, so the above feature could present a greater hazard to those working around trains such as traincrew/shunters etc.
A good point, but a shoe is easier to avoid than a rail, and we're again talking about a few extra inches of clearance.
Seems to me you would need a stronger structure (i.e. more material, perhaps more frequently, thus more costly) to hold a rail against a sideways force when its mounted on the sleeper below.
I did think of the issue of lateral loads, but i figure it's not going to be much relative to a 3rd rail held fairly securely in place, since it only has to hold as much force as you need to hold the shoe flat; plus bottom contact also technically requires lateral some measure of lateral bracing just to hold it up with the "reach-around" supports. Perhaps though there is value in simply eliminating lateral forces as an engineering constraint, even if it's slightly more complicated as per bottom contact.
The shoegear for top and side contact is likely to occupy the same space envelope, so a train capable of using either system would probably require both sets of shoegear to be retractable. So once a top contact system is in use, extending or converting it with side contact becomes very complicated. Similar would apply to bottom contact.
This is true, but i was talking the ability of the
rail to be retrofitted between top and side operation relatively easily, at least not much more difficult than a modernisation overhaul, specifically because they occupy the same space envelope. You couldn't have through running during the overhaul, but it'd make sense as something to bring nearby systems into interoperability, and this happened on the Bury line in 1918, the route to Holcome Brook was apparently electrified with 3.5kV DC OHE until being converted to the side-contact third rail, which must have been even harder. They used to do this kind of thing all the time back in the day, swapping between electrification systems.
You could presumably get most of the benefit from shielding top contact in U shaped plastic troughs?
Oh you could, but people have still built bottom-contact systems in recent years, so there's something there. The main advantage I can think of is weather protection, as ice and debris doesn't tend to accumulate on the side or bottom of the rail. This is one of the main reasons non-top-contact systems were built in the olde days, and is also one of the reasons why modern top-contact insulation generally arches over the top of rail, leaving a gap for the shoe but still covering the top. Still though, as I said, bottom contact seems to be the vogue of more recent years over insulated top-contact, so there's something there. Perhaps it's the possibility for debris to accumulate under this cover? In that respect, side-contact can't suffer from debris getting trapped *under* the rail either, although I imagine that's more than quite rare, apparently bottom-contact can become vulnerable to snow accumulation. Plus, side contact doesn't need troughs and arched covers with drainage requirements, and which would probably accumulate debris, just covers affixed to the top and outer surfaces with small overhangs for effectively the same level of insulation, so long as you don't get between the running rail and the inner contact surface while the rail is live, which is about as easy as not standing on the top of the thing as a workman and much better for foreign objects both human and animal.
This sort of thing seems to be often suggested when discussing third rail. Apart from the maintenance burden of adding all that, it prevents use of the short circuit bar to effect an emergency isolation and would complicate negative strapping for engineering work.
I'm no third rail engineer, but I feel side-contact would be no more difficult in these respects than bottom contact, at least.
Overall I'd think that side-contact just seems to be a slightly simpler version of bottom contact, at least the idea you'd come up with first, so I feel it must have some disadvantage relative to the latter.
One extra problem I've thought of which hasn't quite been brought up is tolerances. The bogies that carry the shoes tend to move more side-to-side than they do up and down, which probably makes it more finicky to maintain contact with the side of the rail than the top or bottom. The Bury line was a sparky *******, for sure, although the German system doesn't seem to suffer here, so perhaps it's not too much of an issue, although maybe it was an extra cost in construction for them.
Also, perhaps we're getting it all wrong; perhaps bottom-contact is just
cheaper, even than top contact, with our modern engineering techniques?
