Did early MTH engines have a switch on undercarriage, selecting btw horn and PS1?

No, I am not aware of any such system and most definitely not such a switch.

DCRU (DC Reverse Unit) + digital horn was one package

MTH PS1 was an entirely different package.

Again, you have basic DCRU, and then a digital horn board plugged into and over the top of this board.

You cannot populate a PS1 upper sound and control board, into a DCRU- and certainly not with the jumpers in place making it a DCRU VS PS1 lower controlled board. Further, there are additional voltage regulators and the battery required for PS1 operation- not present in a DCRU digital horn engine.

Again, DCRU is a different board. It can be identified by the large barrel diodes in the vertical configuration and normally a QSI sticker on the side of the relays. Also note the jumper connecting the DCRU logic that decides forward-neutral-reverse, VS an upper daughter card like PS1 choosing the direction.

Here is an example in a GG1 with a horn board on top of the DCRU in that engine. Notice the horn board has minimal number of vertical contacts to the lower DCRU board. Also the top is relatively flat with only the blob chip. There are different versions of horn or whistle boards, but in general the size, shape, and the fact it plugged into the DCRU this way is all common.

This is a very rare example of a 3 board MTH setup, I believe just called protosound, not yet PS1. The small middle board is those extra voltage regulation sections to facilitate the upper protosound logic board.

Then later, this is your more traditional Protosound 1 board set

here is that lower board, that you can see is related to the earlier QSI design, but also has significant component changes. Notice the large capacitor for electrocoupler firing, the capacitors in the middle between the pin row sockets.

Another physical switch:

There are some early PS1 engines that have a battery switch- that disconnects the battery completely when in storage to prevent draining.

Trying to run a PS1 with the battery switch in off probably results in problems.

Thank you for your concise detailed explanation, Vernon.

The 3 board stack is a QSI aftermarket set up. Not compatible with the 2 stack MTH boards. Like Vernon said, the battery cut-out switch was used before a cut-out circuit was incorporated in the circuit. Another oddity of early MTH locos is if there was a fault in the circuit board is the locomotive horn would honk like a car alarm system on and off until the battery was disconnected. Then the circuit board had to be returned to be repaired.

Early PS-1 would actually change directions without a battery and run.  Just sound interrupted.  If switch left on, the engine sounds would restart.  They also had a different software in top board that make them not compatible with later chips.  G