My personal preference is to listen to music using ReplayGain on a per-track basis, so not preserving loudness differences inside albums. There is an unpleasant side effect of this: track changes where the music flows from one track to the next can get an audible loudness step due to different ReplayGain levels. Considering how foobar2000 makes sure to provide gapless track changes already, there should also be an option to crossfade the loudness change due to ReplayGain to preserve smooth loudness changes.
Interesting idea. But these kinds of albums are supposed to be listened with album gain, anyway.
I used to go with track gain in the past (and sometimes had to move track boundaries manually so that track change occurs at least near a zero crossing) but I came to realize it's wrong and robs me of some qualities of the music and now I'm just using album gain and it's all not a problem anymore.
Frank Klemm's comments on track gain (https://hydrogenaud.io/index.php/topic,35030.msg311995.html#msg311995) (irrespective of encoding format). He proposed to define a limited speed for gain adjustment. The control signal for the gain could be processed with a lowpass. Reaper does this to eliminate clicks when faders are manipulated.
I personally never use track gain. Album and track gains for normal music are close anyway, and calmer songs stay relatively quiet, and intros/outros, speech segments or sound effects do not get an extreme boost. I only use track gain to balance poorly mastered compilations and some singles where the hit song is sometimes louder than B-sides. There the gain change should be immediate or at least very fast.
Some, very fast smoothing by default would be helpful with segued tracks, which are not necessarily part of a special concept album, but still blend together for pacing reasons, or where the cue point has not been placed accurately. It seems that these cases get eliminated with modern digital downloads, and the music starts on the millisecond (not good), but they occur on CDs.