Odyssey speed control, conventional? and MTH 4000 transformer

In conventional, Lionel Odyssey speed control is always "on" unless the engine has a switch to turn it off. Even though it is always on, you can dramatically "activate" the speed hold function by getting your locomotive to the speed you want, then quickly raising the voltage. It briefly surges, then maintains the speed you set. There is a way to change the speed setting, but I do not recall the method (it involves holding the bell, I think).

MTH speed control is also always "on" in conventional, unless Whistle-Bell-Bell is encoded in neutral (followed by 2 whistle blasts from the engine) to turn it off (sound volume diminishes). It then responds to voltage changes more dramatically and will not hold a speed going up and down grades. 

I have not experienced any problems with the Z-4000 and Odyssey, but only have two Odyssey locomotives (and a Z-4000)...

My experience, too, is that it is always on (unless you turn the switch to off).

I would read the manual for the specific loco, I recall something about a three-second period or something for some and i know the specifics do vary from earliest to latest - regardless I don't do anything special, I just run them and Odyseey works well, except the terrible Odyssey surge you can get up in early systems.

As to the "slow speed effect" not sure what you mean but nothing you can do can get locos to creep at slow "legacy" speeds in conventional.  Practically, 15 mph scale is probably the lowest speed you can depend on a conventional runner to work at smoothly while pulling a load.

One fix for the older Odyssey locomotives is to install the ERR Cruise Commander M in place of the DCDR-S board, cures the Odyssey lerch and the low speed performance.

I have two RS11 NYC units that do have the Odyssey but no switch but in the manual on page 8 it explains the the Odyssey mode in the conventional mode.

1.While running at the desired speed for approximately 5 seconds.

2. Press and hold the horn button.

3.While hold the horn button, increase the track voltage by at least 3 volts or a 1/4 of a turn.

4. Speed control is set. Engine will increase slightly then return to set speed.

Turn off

1. While the engine is in Neutral, turn the controller up to the max power wait 1 second then press the and hold the horn button.

2. While holding the horn button, slowly, reduce the track voltage to 1/4 the full power.

3. Release horn.

4. Cycle the engine to forward/reverse. The engine is now out of speed control.

The Z4000 work great no issues. In the command mode you must bring up the handles to 18 volts to get started.

I hope this helps you.