the most important thing to know is the scale:
Bachmann uses 2 scales on "G":
the LGB-compatible 1:22,5 (Gauge 2m) and the more american style scale 1:20,3 (Gauge Fn)
The "standard" Bachmann "G"-Line is 1:22,5 and fits (just for the dimensions speaking) to LGB. Those cars are about 40-50 €.
The "Spectrum"-line is mostly 1:20,3 and is of higher quality and detail. the cars are about 100-150 € each.
I had some 1:22,5, but i changed to 1:20,3 because those cars are simply better and better looking. Contra is the price. Pro is that AMS has also 1:20,3 cars from about the same costs and quality.
More pro is: the spectrum Locomotives are not bad and its really a great fun to have them. I think, they are relatively cheap.
More contra: nearly all Spectrum (not the shay and heisler) and AMS needs 120cm (4 feet) RADIUS. 8ft. diameter.
When u are up to play a bit arround in the garden: take 1:22,5 what is ok and much cheaper.
When u tend to go into the US theme, go to 1:20,3 and make slow: less is more, and when u are going deeper and evtl. have to change (like me), u will cry about every coin u spent on 1:22,5!!!!
Take a look at the pictures section, there u find (by today) my bachmann-spectrum (1:20,3) Consolidation and AMS-Cars.
I have Bachmann-Spectrum cars too. They are great, even though i will never understand why they mount such a big and dump coupler.
Frank