I'm unaware of any obstacle other than will and money.
But I don't know how much geo-tech has been done, if any, exploring the possibility.
@smallspy is more likely to know.
To be clear, under-pinning an old tunnel is theoretically straight forward, but it is, at best, time-consuming and somewhat costly. Depending on soil conditions, and water table height, and what, if anything is underneath.....it could be fairly involved.
It also requires re-grading the trackbed, for a material distance beyond both ends of the tunnel.
I was being a bit cheeky about how easy it is; but at the same time, it shouldn't be all that insurmountable.