If you look at the real cost-benefit of a Tier 4+ - people's health v. cost to the corporations and overall economy - it's actually pretty good. But again, in the current administration, that doesn't matter so I can't see this going anywhere and it could actually cause backtracking on already existing standards.

Emission reductions up front reduce health care costs over a long term, not immediately, so it's logical to pay for it over a similar term. Public funding is needed for that, because the payback period is much longer than can be accepted by commercial financing, especially in the amounts needed. That's why electrification is at least partially publicly-funded almost everywhere it's done. Even Penny's electrification used some public funds.

Electrification in the U.S.: if diesel can be further cleaned up, it makes electrification dramatically less cost-effective for freight lines. Even at Tier 4, if it were universally implemented (takes time for the fleet to turn over), it's hard to make a good argument for electrification on health grounds except in hot spots near yards or very high-traffic lines, and the operational issues with tiny islands of electric in an otherwise diesel system are significant. Freeways and major road intersections with a lot of trucks have similar issues, which is why truck emission standards will almost certainly go beyond Tier 4 eventually. For passenger service, electrification still might make sense for operating reasons (see: Caltrain) and certainly makes sense for high-speed (above 120mph/200kph) service given its power demands. Yes, some railroads in the U.S. did electrics without public funds, but those lines were special-purpose islands that are long gone, eliminated by demise of the railroad or technological (diesel) progress.