Would be great to see that for what it would mean for US rail in general. If you’re referring to the Gotthard Base Tunnel (the newest Gotthard tunnel), it was justified on the basis of 200 or more freight trains and 50 or more passenger trains per day. Even at that I wonder what the payback period is. It was also part of a realignment of the continental rail freight system. I don’t know if it was only Swiss money that built it or if there was substantial EU participation.
To provide a very brief summary of what was a very convoluted and long story:
One of the motivators behind the tunnel was the heavy truck traffic that was congesting the Gotthard highway and causing noise, pollution, and degradation of the highway, while simply using Switzerland as a country of transit and contributing very little to the local economy. The Swiss were very unhappy about this and it became a major point of contention between Switzerland and the European Union. In an attempt to discourage these trucks, the Swiss had imposed a weight restriction (of I think 28 t, vs the 40 t that was the European limit at that time), a restriction that the European Union considered excessive and were extremely unhappy about.
The European Union believed the inability to send larger trucks through Switzerland was crippling economic opportunities and was an obstacle to the economic integration of Southern Europe. The Europeans wanted Switzerland to remove these restrictions and massively add capacity to the highway, but the Swiss refused. Finally the European Union came around to agree with the Swiss proposal to build a rail runnel instead (a project that had been floating around in some form or other since at least the immediate post war period). In return for this, Switzerland would accept heavier trucks, but would levy a distance- and route-based toll that all trucks would have to pay, with a significant part of the money raised being used to finance the tunnel. The levy thus has two purposes, it discourages trucks by making them expensive, and it raises money. Initially the European Union said no way, but after more negotiations and some compromises, they basically accepted this proposal. Every truck is equipped with a device that uses GPS data to measure distance travelled on Swiss roads, and which roads are used. Cross-Alpine routes are charged significantly more than trucks doing local deliveries and distribution. The Swiss put the construction money up front, with the money from the truck levy being being used to repay the debt. AFAIK they are still repaying today.
So the short version of the answer is, the truck companies (and by extension the consumers all over Europe) are paying for the rail tunnel.
