Well, they're arguing that your claim is nonsense. Here's the reason why different countries ended up with different standards for various railway things: interoperability simply wasn't that big a deal at the time. These weren't continent-spanning high speed train services, they chuffed along at a speed of 30-40mph and had frequent stops because the locomotives needed regular watering and coaling and the passengers needed regular watering and emptying as well (no on-board toilets or restaurant cars yet). Border crossings usually involved a lengthy stop while formalities were completed, and if a train was crossing the border they'd simply do what happens right up to the present day in many cases: change the locomotive for one belonging to the company that operated the railways in the country they were entering, staffed by drivers who knew the local rules, signalling and practices.

I know this doesn't cover for breaks of gauge, but they were handled in a similar way -- border stations were simply connected to both systems, so when crossing from (say) France into Spain passengers would alight, clear immigration, and board a new train on the opposite platform to take them onward into Spain. The French train would then usually (all going well) return into France taking the passengers who'd left the train from Spain (which falls mainly on the plain) when it arrived there. Freight was obviously harder to transship, which is why at least initially the railways were more interested in enabling through-running of goods wagons without having to offload and reload the whole shipment than they were in through-running of passenger carriages.