The general strategy would be a) introduce less heat and b) extract more. The first might be accomplished through greater efficiencies, though that's limited.
If a major heat component is braking, then locating the additional cooling capacity where breaking is heaviest (presumably on inbound station approaches) might offer advantages -- at the very least this reduces the total treated area for maximum effect.
Given the possiblity of ground-based thermal banking, and the long-term nature of the issue, if any amount of coolant could be circulated through the thermally-affected clay, and made available for seasonal heating needs elsewhere in the city, that might be a net win.
I'm familiar with geothermal energy projects elsewhere (borehole projects in Australia, the Habernero project) where the problem is actually inverted: themal extraction cools the strata around a borehole, over the course of about 40-50 years, to the point that no further useful heat can be extracted.
The thought also occurs that the steel rails themselves are thermally conductive and might be made a part of the cooling system. Not a tremendous radiative surface, but a long conductive length. Poorly placed, that is, low within the tunnel, rather than high, for effective heat extraction though.
If a major heat component is braking, then locating the additional cooling capacity where breaking is heaviest (presumably on inbound station approaches) might offer advantages -- at the very least this reduces the total treated area for maximum effect.
Given the possiblity of ground-based thermal banking, and the long-term nature of the issue, if any amount of coolant could be circulated through the thermally-affected clay, and made available for seasonal heating needs elsewhere in the city, that might be a net win.
I'm familiar with geothermal energy projects elsewhere (borehole projects in Australia, the Habernero project) where the problem is actually inverted: themal extraction cools the strata around a borehole, over the course of about 40-50 years, to the point that no further useful heat can be extracted.
The thought also occurs that the steel rails themselves are thermally conductive and might be made a part of the cooling system. Not a tremendous radiative surface, but a long conductive length. Poorly placed, that is, low within the tunnel, rather than high, for effective heat extraction though.