In short, how on earth do you connect passenger and freight cars to an engine?

Everything I have tried has failed and keeps coming up with the error message shown in the title.

Never thought about that, but I think it is not possible.

In a trainstation you have different platforms for passengers and cargo.

You could try to place a passenger platform on one side of the stationtrack and a cargoplatform on the other side, but I doubt that this will work.

I use different tracks for cargo and passengers. Just look at the speedlimits, makes most times no sense to mix them.

Adamant102

Thank you but I didn't explain myself all that well I'm sorry. I meant, how to connect either freight or passenger cars to an engine to form a train, not both types of cars to one engine.

I did figure that part out though, now I can't seem to "ADD" additional cars to a train that is in service. IE I have a passenger train with 4 Passenger cars BUT want to add another 2 cars to that same train??

You can do that in the vehicle manager.

Select the train, click on the tool symbol, add some cars, then the shoping cart to buy the added cars.

After that you can close the vehicle manager and your train got some more cars.

If you forget to buy them, nothing will happen.

The vehicles does not need to go into the depot, you can change the vehicles everywhere ( Lorries, Busses, Trains, Planes, Ships ).

Thank you Adamant102, appreciate the help.

Doing Cargo and Passenger in the same train is not possible at the SAME STOP (Platform doesnt matter).

Cargo & passenger station is treated by the game as 2 different stations.
It does work if you do 2 stops. Advise the line to stop once at the Passenger station then on the Cargo station (symbol) seperate.

BlackHeroe

Once again, thank you.