Hi Eino
Yes on some (older) Märklin locos, there are wheel sensors in the form of a cylinder on one axle which is magnetized following 2 axles at a 90° angle (so to have N S N S in a full rotation.
This was used to generate the "chuff chuff" sound synchronized with wheels.

Example the first BR45 (37450) or the first Big Boy 37990).

Doing it that way lead to a simplified sound and additional cost (the magnet on one driving axle and the Hall transistor (sensitive to magnetic field).
Now it is made by synchronizing the wheel speed with the sound to reach 2, 3 or 4 strokes per driver rotation
My experience is also, the wheel sensor sometimes, does not work well with multiprotocols after all these years (My CS3 for instance). The likely cause is inducted electrical noise in the Hall transistor line.

Regarding squealing sound, it never had to do with a wheel sensor but a rather complex logic:
- must reach a set speed before any slowdown will trigger squealing sound
- a CV to set which speed it starts squealing
- other CVs.

Regarding AUX modulations (fire, blinking, Mars light, Telex, etc) it never had to do anything with a wheel sensor. All is done by the computer in the mSD3 or mSD2 decoder
I don't know much about ESU as I have only one LokSound 5 micro.
Cheers
Jean

I think those features are implemented in software on ESU locos and they do not require wheel sensors, although I could be wrong.

I have one beautiful ESU V200 with reliably squealing brakes both when slowing and in curves. The sparking is super cool and it is one of my favorite MRR tricks ever.

It also has lighted instrument desk, cab and engine room lighting, and dynamic diesel exhaust. ESU has been making locos like this for years.

Hi
Both ESU and Märklin decoders have inputs.
To detect a curve position, what is needed is a curve detector (bogie turned right or left and not straight) and NOT a wheel detector. Then the squealing sound may be triggered when the loco is moving and when the sensor is activated (no squealing when the loco is stopped).
This part is for sure.

How does ESU detect a bogie not in straight position? I don't know.
One simple and reliable way, I think is to use a small reed sensor. They have the particularity that, with a proper magnet orientation, when the magnet is right in center of the Reed, the contact is open and as soon it goes in one side or the other, the contact closes. This is when the magnetic field is perpendicular to the Reed axis.
The magnet must be small, away from the motor not to distort the magnetic field and attached to one of the bogies (or pony truck for a steamer).

Cheers
Jean

If I should guess, you can probably make something similar to the esu with an msd3. As you say put one or two sensors on the bogie and the reader in the centre of the chassis and when the bogie rotates a sensor input is triggered, playing the sound an flashing the led. but there is a big if and that is the Marklin decoder and getting compatible parts, yes the msd3 has sensor inputs and yes they work but it's not documented. But I managed to get the wheel sensor on my big boy to work perfectly fine on an msd3.

With a mSD3 decoder, it would be on side of the Reed to the decoder's input and the other side to the frame
*** Edit ***
Checking again on both Märklin and ESU I found Märklin does say a word on its input coming to Pins 1 and 2 of the 21MTC interface.
On the LokSound5, page 91 of the manual, it says up to 5 inputs are available but not in all configuration.
With MTC21 one "wheel sensor" is available and can condition fonctions.
Some digging on the LokProgramer is advisable.
The advised sensor is a Hall transistor connected to 0, +5V and the input (see page 34 of manual below

LokSound5 manual
Cheers
Jean