Do those locos ur using there come with the latest ttd patch?
They look pretty ok.
Basically I buy a helper loco ,it spends it's life as a helper ,it can't pull a train by itself ,When I play I don't scrap locos when an industry closes and I gotta rip up the rail link to it ,I store the loco ,but if I'm using a helper then that helper cannot go to the depot by itself ,it is immobile without a lead loco!
I try to base my ttd games on practices in real life ,shunter locos integrated networks and so on. I allocate locos ,I don't build them to stay on one service forever regardless of wether it's viable or not.
Here's an example
I build a DEACCW loco for a coal service ,It doesnt have enuf power ,so I build a helper.
Then the production falls ,I take a few wagons off the train to reduce loading times then I decide I don't need that helper anymore ,however I can't put the helper loco on a new service to save the cost of building a new one cos I can only use it as a helper.