Creating a Consist with the Activity Editor

Open the Activity Editor, from the Train Simulator Editors & Tools icon.

From the File menu, select New, and the Select a Route box opens. It doesn't matter which route you choose, as the Consist isn't route specific. However, if your locomotive is electric, then it will only be available on an electric route. For example, notice you are unable to select the Acela if you are on the Marias Pass route.

Then your route map will open. See on the right of the window, the Player Service? Select New.

The Service Editor box. See, about halfway down, it says Consist? Select New.

The Consist Editor box. Starting with the upper right side, fill in a Name and Display Name. The Name will be the filename, and the Display Name is what you will see in the box, when you go to choose this consist.
Every drivable consist must have an engine. Go ahead and choose your engine. Whatever it is that you've just downloaded and want to test, or one that you like to run.

I like my model of Sandersville Railroad #400. Notice a couple things. While not evident here, the list of locomotives, or cars, isn't necessarily in Alphabetical Order. You can click on Name, at the top of the list, and it will sort them. Also notice the preview box of my loco. In this picture, half the locomotive isn't there. This in itself isn't a cause for alarm. Several newer models have two shape files, but this editor will only display the base model.
You can drag and drop the locomotive from the preview window to the consist window. You can repeat as many units (or cars) as you want, though I think there may be a limit somewhere above 75 units (or cars).

To flip a unit, then right click on it.

To remove a unit that you don't want, then drag and drop it to the odd square shape on the left, just above my first unit. I think that's supposed to be a coupler.

When your consist is how you want it, click Save & Exit at the bottom right of the Consist Editor.

Cancel from the Service Editor window, and exit from the Activity Editor. If it asks you to save, which it shouldn't, then you should click no, unless you were designing an activity.

Load it up. Choose your route and Engine, like you already know how to do.

Look for your Consist name, and click the Start button.

There you go.