Open Metrowars in firefox and log in. Click Home to get to the stats page. Right click on any open space in the window and set it to reload every 15 min.

Wait for your Energy to hit 100%

When it hits 100%, right click over the "gym" link and open it in a new tab.

Select the attribute you want to train, and hit train. When the results page comes up, right click anywhere on an open spot and set it to reload > custom. Set it to reload about every 40 min.

Result:

Two tabs in one open Firefox window. The first tab keeps you logged in. The second tab refreshes the train php in 40 min when your Energy is back up to 100%.

As far as the game goes, it's not illegal. It's probably not encouraged though. To avoid unwanted admin attention, I would only use the auto train at night when you are sleeping.