But neither of them are electric. I wonder how much of a reduction in noise (especially low frequency hums) these EMU trains can provide.
A huge amount. It's not just the overall sound pressure level that drops, it's also the composition of that sound.
Example: An opera singer and power tool could register the same dB level, and one is far 'sweeter' than the other. 'Loudness' is a good part of that.
Good paper here:
https://www.acoustics.asn.au/conference_proceedings/AAS2013/papers/p99.pdf
No they are not just track noise, they are mostly low frequency diesel engine noises which can be so much louder (more than a squeaky TTC train!).
That's a number of factors at work, one of which is that low frequency and infra sound travel much more freely and further than high frequency sound. Marine fog horns work utilizing that principle. In the case of the UP train, your location as well as the source are factors in perceived sound pressure differences. Low sounds are uni-directional, higher ones tend to beam, and I too have noticed a low freq resonance to the UP exhaust system. Some of them chortle, and I find it pleasant, but some of them sound harsh. I'd suspect that's an indication of an engine or exhaust system problem. The exhausts on them are complex to scrub the emissions, it may be that which is exacerbating it.
You'll notice as well that when it's damp (the 'air is heavier') that low frequency sound propagates more readily due to the atmospheric density increase. That tends to 'damp' higher frequencies, and assist propagation of the lower ones. Again, think 'fog horn'. And there's a third factor when the air is damp and denser: The resonant point of a given system shifts down, as much as 20% in tuned systems, and the 'coupling efficiency' of a source to atmosphere goes up too.
With modern electric propulsion, almost always triple phase induction type motor, the greater sound is from the 'chopper' than the motor's performing work. That's mostly due to inductive core lamination vibration, both transformer(s) and motor laminations, and it's that 'space-ship' sound in the audible mid-range that you hear. You can tell the type of motor control being used by the nature of that sound. Some are a lot quieter than others, some remain the same frequency but the pulse width is modulated, others successively climb in pitch and then drop again like a car's transmission does for engine revs, but it's almost always far quieter than internal combustion.
And sweeter.
