Electric is as sustainable as its sources today.
Smartville has 80% coal 20% large hydro (which lends itself to pumped storage). They are replacing 5% of their coal with renewables every year, increasing hydro backpumping capacity to allow pumped storage. They buy a bunch of electric buses/trains. Over the 32 year life of the trains and buses, their renewable percentage is what -- anyone? anyone? Bueller?
80%. The 16 years of phasing out coal started at 20% renewable rising to 100%, averaging 60%. The next 16 years were 100%. Averaging 80%.
Dumbville has the exact same power and conversion-to-renewables plan. Since their electricity is 80% coal, they determine that it's not worth making the trains and buses electric since it'd only save 20%. Over the 20 year life of the trains and buses, their trains and buses' renewable percentage is what?
0%. Because they remain diesel, even as the power grid cleans up.
Also Dumbville needs new trains and buses at 20 years instead of 32.
Electric trolley buses are a thing.
Watch that video and pay close attention to the grades. Trolley buses eat hills for breakfast. While a diesel bus is lugging up the hill at 10 mph in low gear, the trolley just pulls big motor amps and climbs like a rocket.
Since the location of hills is not a surprise, the transit agency makes sure substations and feeder ties are plentiful in that zone, so the bus is pulling at the ~600 amp limits of a graphite contact shoe. Generally my experience is trolley buses are significantly faster than diesel buses, and more agile in the urban environment. They have no trouble spooling up to 50 mph, so you have a very wide operating range without any transmission.
5 US cities have them (the video was American)
The longest and most "intercity" trolley bus line is in Crimea, from the middle of the province (Sinferopol) to the shore (Yalta).
Europe is covered with these systems, and both Western Europe and the Russian sphere have ample demand to support enough builders to keep things competitive. Not so in North America; there simply aren't enough systems to keep one trolleybus builder alive let alone two, so every replacement job ends up being bespoke with very complicated design-build-accept process with a high learning curve and lots of expensive blunders.
There's no way to set up a company that supplies 10 buses a year to each of the five cities, and constantly improves its product and optimizes production and drives down costs; because that's not how Federal matching grants work. The Feds pay a one-time, 9-digit bolles of money, and they demand you get all your buses in one big batch. That causes a feast-famine cycle that makes it impossible for any manufacturer to stay in the business. Simply changing the Federal law to be anti-bolles and require continuous production streams, would reboot the industry.
Batteries are changing everything for trolley buses
Since about 1993, batteries started providing a useful function: allowing the trolley buses to roam off-wire, to get around obstructions and the like. The idea is they could roam a few blocks around a construction site, and replenish the battery during the rest of the route.
But now we have Teslas running around with 100KWH battery packs that can punch 1150A. It could definitely power a trolley bus for a little while if you could keep it cool (or use a larger pack). But could it make it through a 20-hour workday of one of the buses in the video? I doubt it. So for serious, highly kinetic urban transit like you saw in the above Youtube video, a pure battery-electric bus is thermodynamically impossible.
But if you look at the map of many trolley bus systems (and for that matter, diesel or rail systems), you see a lot of places where 2-4 lines double the same route for much of their length. For instance, Haight-Ashbury in San Francisco is a choke point which causes most of the service in the northern Sunset (6, 66, 71) to funnel into Haight Street and run under the trolley wires all the way downtown. But the 66 and 71 soot up those wires with combustion smoke; their routes have no wires. Well, with a large-battery trolley bus, they can simply pull the poles down at the end of the shared route, and motor into the 'burbs on battery. They only need to do that, and the inbound run to make it back to wire to recharge. They could even have isolated wires at the outer terminus, so the buses put their poles up there to recharge while they wait for the next departure time.
The cars could also have solar panels on the roof for a little more help (and self-recovery if a bus goes completely flat; lock it up, come back tomorrow and limp it home). For that matter, they could install a small diesel APU to help replenish the battery day and night.
And for that matter, they could have a large diesel APU that replenishes the battery always, in all modes of operation. Now you don't need the trolley wire at all. On the other hand, you just went from 100% electric to 0% electric - all that tech and we're right back to square one. And that is starting to look like where trolley bus is going. This is the root of the "electric bus" problem - you're a diesel APU away from completely undoing all the advances.
Electric trains vs electric buses
So now we get to the core of the question. But we can't avoid addressing land use.
- Building a train line is high infrastructure, but it also spurs development along the train line. Every high-density condo near the train station is a house not built out in car-land (the untransitable fringes), so that's great for sustainability in the long term.
- However, the electric bus, if we use some trolley wire to solve the "thermodynamically impossible" problem, has the ability to easily expand out into areas now poorly served in the short term. The downside of this, is that "car-land" is usually very low density, which means the bus has a lot of swept area to collect a few passengers, so you don't get much bang for your transit buck.
The problem is, aside from pure battery-electric buses not being possible, that the "augmented trolley" I discuss is also not a thing in the scale I propose; certainly trolleys have 3-mile battery packs, I'm proposing 20-mile. It could be a thing, but it's stymied by that production-stream problem that prevents stable businesses in the field.
The other thing is that trains are simply not possible in a wide variety of locations. It's very hard to punch a rail right-of-way through virgin territory. Most rail builds are laid in streets (Detroit M-1 Q-line), on abandoned rail lines (conspicuously, Los Angeles' Pacific Electric "Red Car"), alongside existing freight railroads (WMATA), or in the median of freeways expanded for that purpose (BART).
Trains also require a density that is not feasible in most of "car-land". Whereas electric buses are perfectly viable in lower density places like Dayton, Ohio, that are far, far, far below the density required for any kind of rail.
So, electric rail is a turn-key solution, but only in a few places. Electric buses have wide application, but need to be developed.