The key to success is whether the railway company can acquire enough cargo that makes the most of the characteristics of the shinkansen to cover the development costs of the dedicated vehicles.

All you need to do is not install seatings and enlarge the doors. Done. Ask the airline guys

I'm pretty sure this is not an "all you need to do" situation. The stations are not set up for freight. The freight they are talking about is high-end produce, so this means refrigerated cars. If one area's farmers/fishermen get this, expect every other area to push for taxpayer spending for them to get it as well. The Japanese countryside has many regional airports built for this reason. Some are completely unused.

Robert. Not at all that simple. Even if it were as simple as converting existing car designs, which it isn't, you'd need additional new lines and terminals dedicated to on/offload the cargo. Procuring additional real-estate for all this that's connected to the existing shinkansen lines would be a nightmare and probably cost prohibitive.

Infrastructure of benefit to the economy is usually subsidised by the state. And the alternative to this will be not having stuff and not being able to get stuff to customers.

Japan has a fabulous rail network and tweaking it to move goods long distances is possible. You aren't moving stuff from central Sapporo to central Tokyo, you are moving stuff the majority of the way there, so your freight sidings can be anywhere convenient at either end.