[feitingen]

> The interoperability benefit is limited since you typically won't want to operate both Shinkanzens and local trains on the same rail.

> Having different gauges provides no real benefit.

The only place it would be beneficial to have the same gauge would be at stations, to have fewer, more flexibile platforms and smaller train stations.

[The_Colonel]

I can see other advantages like having same maintanence rail cars, emergency rail cars, ability to divert traffic in case of accidents. I guess there are even more such minor advantages.