The thing is, the trains in the United States that use long tunnels really aren’t going all that fast anyway - 79MPH being the upper speed limit in 90+ percent of the territory. Add the fact that tunnels usually occur in mountainous territories, which limit speed by grade and curve gradient, and it’s just not much of a problem.
On the other hand, trains in most of the rest of the world that travel through tunnels are already electric - I’m thinking specifically about the Eurostar that runs through the Chunnel. In fact, I’m not aware of any high speed train in the world that doesn’t run on straight electricity (jets and various turbines have been used, but that’s never really been more than experimental.) I don’t know enough about the TGV or Shinkansen to know whether they travel through many tunnels, but both are electric.
Some (most?) of the larger tunnels in the US have exhaust systems to clear the diesel fumes - both for the benefit of the next crew as well as the diesel engines, which obviously need to breathe. So it’s not as if the two portals are the only intake/exhaust ports. Quoting from the current issue of Trains magazine (October 2007): “…Stampede Pass [Washington] had one key thing going for it: The 7.79-mile Cascade Tunnel at Steven’s Pass must be ventilated after each train passes, a process that takes at least 30 minutes.”
In short, I’m guessing that no, the relatively minor increase in efficiency wouldn’t justify the cost of powering it - either we’d electrify the railroad (which would then put us on par with Europe, but since we have hundreds of thousands of miles of track, it’s not feasible) or we’d modify the diesel-electric locomotives to supply the power (or, I suppose, have each tunnel supply its own power by some means.) Then, GE and EMD would have to offer that option on locomotives - an expense - that only a few power divisions would need (the mountain divisions with tunnels) which would be subsidized by all the other railroads/divisions that don’t need the modification.
Also keep in mind that many tunnels are at grade - so (for example) the Westbound train would be using the diesel engine to load amps to the traction motors, effectively ‘working’ to get the train through the tunnel, the Eastbound train would be going downhill, probably in dynamic braking, and would have no need to displace air in the tunnel - that train is trying to decelerate.