Adding Wagons and Carriages to existing trains.

[Didz]

Is there a clever way of doing this?

I currently have a passenger line (The River Valley Railroad) with four trains running on it and a line frequency of 12 minutes. So, I don't really need more trains, but at the moment each engine is only hauling three carriages with a maximum capacity of 48 people, and some of my stations have over 80 people waiting for a train.
http://steamcommunity.com/sharedfiles/filedetails/?id=1190458202
So, I'm looking to add another carriage to each engine. But whats the best way to do it?

Sending a train to the depot results in the loss of the entire passenger revenue, as it seems that the passengers just get ejected at the depot and left to walk to their destination.

To try and avoid that loss of revenue I've tried creating a dummy line. (The Depot Line), which basically only has one station on it (Elizabeth) and by waiting until a train is bound for Elizabeth I can switch it to the Depot Line, and it will complete its journey to Elizabeth and empty its passengers and then sit there waiting for instructions as it has no other destination.
http://steamcommunity.com/sharedfiles/filedetails/?id=1190467041
The key point being that because it isn't going anywhere it doesn't pick up any passengers. So, I can then order it to the depot without losing 48 paying customers,
http://steamcommunity.com/sharedfiles/filedetails/?id=1190473646
http://steamcommunity.com/sharedfiles/filedetails/?id=1190477729
But is this the best way of doing it, or does someone have a better/easier solution?

[Skifans]

Its likely to be more expensive but you can also use the line auto replace function. Usualy I go for a middle ground option, don't bother with a dumy line and wait until the train I want to add a carridge to is at a station, wait for it to unload but try and click the send to depot button before it commences loading. If there are only two stations on the route and you have instant reactions this should result in no lost passengers. If theres more then two stops you'll still lose some of the passengers, so I try and do it at one of my busier stations.

[Kilroy115]

Yeah, for my main line passenger service, I normally place depots at the end of track station, at both ends of the line. Then I just catch the trains right as they finish unloading passengers before they can start loading and send them to the depot.