What's the most efficient way to replace old locomotives in OpenTTD
Solution 1:
There is an option to autoreplace trains when they go for servicing. Go to the Trains menu and select the 'Replace vehicles' button (this is also possible for all other types of vehicles)
Then choose the train you have currently and its replacement, and select the 'Start Replacing Vehicles' option to start autoreplacement.
When all vehicles are replaced the old vehicle in the autoreplace list will be greyed out, and you can choose 'Stop Replacing Vehicles' to remove it from the list.