Maybe someone can enlighten me?

I understand the need for electrically isolated wheels on 2 track systems, and fact that non-isolated wheels are required for some track detection methods on 3 rail layouts.

Are there other not obvious reasons why both options exist?

The 3 rail system gives a much more reliable electrical system, and also prevents the need to have reversing loop isolators.

True, but wagons and coaches with NEM DC wheelsets are more likely to run on Marklin C track (and even M track) than an RP25 DC wheelset (European v's American standards). I have had no trouble running Trix wagons with NEM DC wheels on my Marklin M track.

Yes, the AC are more "rounded" and the DC is more "flat".
Also distance between flanges are different. Around 13.9mm for AC and 14.1mm for DC. This small difference is all that separates a de-railing on every turnout from a clean run!

Usually, if I dont need the electrical connection between wheels, I just squeeze the DC-wheels together to get the right distance. Some brands even have two tiny-small grooves on the axle, which marks the right settings for each system .

Edited by moderator 06 June 2011 21:24:00(UTC)  | Reason: Not specified

In the 2 rail DC system, each rail has one pole (+ or -) for power transmission and isolated wheelsets are required to avoid a short circuit. Loks, coaches and cars historically had power pickup from one rail on one wheelset/bogie and from the other rail and the opposing wheelset/bogie. More recently, companies have started to use wheel contacts that enable power pickup from both rails on the same wheelset/bogie.
3 Rail AC originally involved metal track sections, with the main being transmitted along the middle rail, with the outer rails being used as return. This meant that there was an optimal connection between rolling stock and the tracks. The more modern K and C Track have plastic bases. The rails on C Track are still connected to each other via contacts on the underside of each track section. K Track is essentially the same as DC Track, except that the center rail studs are present. The contact between rails exists only on power connection tracks, contact tracks and perhaps on switchtracks. K Track can be used for 2 Rail DC operation if properly wired.
Using a unisolated wheelset on 2 Rail DC will result in a short circuit. isolated wheelsets (DC) can be used on both DC and AC layouts.
Maerklin developed H0 and established the spacing between rails. Other companies later developed a new profile that was a little more exact and that profile was adopted by most of the American companies and later by the Asian and many European companies. This is currently the DC standard.
It is important to remember that 3 Rail was also used initially by Trix (Trix Express), so Maerklin was not the only company having a different standard. Fleischmann also had it's own rail profile which was not fully compatible with other DC makes until the 1970s.
Today's technologies offer companies the possibility to manufacture loks and rolling stock where the wheel spacing and contacts can be easily adapted for AC or DC operation and in the future, it is likely that models could easily be switched from operation on one system to operation on the other. RailTop announced a few models with this in mind, but none have yet been commercially released. Bemo currently offers it's models in switchable H0m to H0e (narrow gauge) so that shops no longer have to stock two versions of each item.

Regards

Mike C

Hi,

Just buy AC, the system you set up will run, if you use C track you will be up and running fast, enjoying your trains. Plus you can run both AC & DC cars behind your engines without a care in the world. AC engines cost more however they perform much better.

Matt

Märklin's AC versions normally have mechanical switches. Those will work with AC or DC, analogue or digital.

So the Trix version could have such a mechanical switch - or simply diodes that can be replaced with a decoder for digital operation.
The description in the Trix catalogue doesn't allow conclusions to be drawn what the train will have.
Explosion diagram not yet available.

I've never heard of control cars that detect voltage spikes for analogue AC operation.

I had some issues with DC wheelsets on C-tracks. I bought the Pacific Fruit Express set (45680) which derailed almost every time when crossing the turnouts. For a newbie like me, I had to do a lot of research before I noticed the difference in the sizes of the flanges. Then I also noticed the insulation. When I pushed the boxcar manually over a turnout, I noticed that the wheel hit the centre piece of the turnout and derailed.

I think it is a bit weird that Märklin ships sets with dc-wheelsets, if they run this bad on C-tracks.

I have attached a comparison between the NYC boxcar from the US starter set and one of the PFE-boxcars.

You are correct on the wheel sets. On the box it says NEM: 320 552 and RP 25: 320 389. I googled the part numbers and found out that both are DC wheels. Though to get the right wheels, I've ordered the 320 551 wheel sets from the dealer (on which I got a nice deal). I didn't notice this on any of the other car sets I bought from the same dealer: There were large flanges on the UP Hopper set (45800), 3x Tank car set (45580) and the UP Caboose (45702).