Gosh Louie, I thought you would have got a reply to this.
I was also looking for an answer.
Maybe someone knows out there.

What Mike said above is correct, but if you're dead set on a Porter, there are numerous articles on the forums on how to add DCC to a Porter.   If you can handle some soldering and modifications then you can have a DCC Porter.

Well, never hearing anything from the Bach Man, I went out and bought a set of each of the 4 freight trucks and measured them myself.
To bad Bachmann doesn't supply this info, especially with the variation I found when measuring them.
So for those interested, the following is what I found.

On30 RR Truck Dimensions

                                                        Actual Measured / Scale Dimension
Item #    Description                 Wheel Diameter   Wheel Base    Bolster Ht.
Bachmann (all metal wheels)                                                   above Rails
29901    Arch Bar Freight Truck      .510 / 24½"     .95 / 45½"      .50 / 24"
29903    Arch Bar Long Wheelbase  .508 / 24"     1.113 / 53½"   .468 / 22½"
29904    Arch Bar Low Frame         .451 / 21½"   1.032 / 49½"   .300 / 14½"
29905    Vulcan Freight                 .497 / 24"      1.02 / 49"        .44 / 21"

As for sound, look at one of the MRC older sound systems like http://www.modelrectifier.com/search/product-view.asp?ID=7614 or for just city/country sounds http://www.modelrectifier.com/search/product-view.asp?ID=1293.
Probably find one cheap on e-bay and this would give you what I think you want.
Dave

That's a good suggestion Bill.   I'm definitely going to look for some of those cars.   Looks like they are a good resource all around.
I still would like to know more about the ones in Bachmann's On30 list.

Thanks Alan, that's a good start.
Under the On30 accessories, Bachmann lists 5 trucks:

1- Low Frame 4'1" Arch Bar
2- Long Wheel base Arch Bar
3- Arch Bar Freight
4- Diecast Passenger
5- Vulcan Freight

It would be nice to know the wheel diameters of these and other frame dimensions so they can be used by scratch builders.   
And do all of these come with Metal wheelsets?
Maybe the Bach Man has some info.

Currently Bachmann offers 6 different trucks.   
Is there anywhere (either on Bachmann or another website) that the wheel diameters are specified and the height of each truck assembly?

I'm planning to build a couple of rolling stock kits and maybe a scratchbuild and would like to follow 1920's prototype as close as possible.
I have the correct Cyclopedia but need to know which wheelsets and trucks are close to dimensions.
I've been unable to find any dimension drawings on the Bachmann website.

Thanks
Dave M.