Len,
I would completely forget about how the MTH Triplex and any Bachmann 1/20.3 locos might look together..
Its pretty much a non-issue..you wont want to run them together anyway..
the answer is "they will look really really weird running together!"
Scale is radically different.
prototype gauge is radically different.
prototype size is radically different.
Era is radically different..
Basically you are asking, how will this:
look running with this:
the answer is: really strange.
they simply dont "go together" at all..
they have nothing in common, except they both have flanged wheels and can run on the same tracks.
but its also your railroad, and you can run anything you want on it!
Just dont think they will "go together" in any reasonable way..because they wont.
but if you dont care about that, then go for it..
In cases like this, most people just run their "widely different" rolling stock in separate trains.
For example, I own USA Trains GP38-2 diesels, 1970's standard gauge diesels:
and Bachmann Spectrum moguls, 1870's narrow gauge steam:
I own both, because I love both..
I will happlily run both on my garden railroad, each with their own appropriate "matching trains"..
but I wont run them coupled *together*!
even though I could if I wanted to..
They might run past each other..which would be a prototypically inaccurate mismatch..
but again, that doesn't bother me..simply because "its my railroad" and I like both locomotives..
They dont "go together" at all, not even remotely..but "going together" is simply a non-issue in a case like this..
thats what I mean by "I would completely forget about how the MTH Triplex and any Bachmann 1/20.3 locos might look together..
Its pretty much a non-issue.."
if you like both, then own and run both..nothing wrong with that..
but "going together" isn't part of the equation..
Scot