I use a command called "Instant Load" this will fully load your train with the commodity's it needs or requires...
To use "Instant Load", find the portal you want to use, then assign trains and drivers to the portal, and in the commands section, before you place any commands on what to do, place the "Instant Load" command in, this will then fully load your train as leaves the portal.
If you don't see "Instant Load" in the commands, go to FILE at the top of the screen in Surveyor, then scroll down to Edit Session, then go to Driver Commands, click on to the driver commands folder, and then scroll down the list of commands until you come to "Instant Load" box, then click on the box, this then should give you the command in surveyor and in driver.
Hope this helps
Joe Airtime