In message , at 16:44:49 on Sun, 11 Mar
2018, Anna Noyd-Dryver remarked:
- They told me that the rest of the railway is still owned and
maintained by the Royal Mail. There is at least the theoretical
possibility of the museum trains doing a longer circuit, perhaps to
the next station, but there appear to be no current plans for such an
expansion. It would obviously need Royal Mail's agreement, and there
would be a significant costs. The current trains may not have the
battery capacity for the extra distances involved.

Problems include lack of emergency exits.

From the site's FAQ:

"Evacuation requirements mean that access to the Mail Rail train ride is
restricted to those who are able to walk unaided on uneven terrain, in a
confined space for up to 100m before climbing 70 steps to the surface."
--
Roland Perry