I am not sure you are still struggling with your render as its a late answer. But may be useful for some one else too.
Its not good to do that manually or set the finished alarm.
I will take back you to 90's where we can set the commands in command prompt in a batch, using batch files.
Lets say you want to render two files you may use it like this
blender -b "D:\Path\Train 001.001.blend" -s 1 -e 2 -a
blender -b "D:\Path\Train 002.001.blend" -s 2 -e 4 -a
The first line is for the blender exe file path, you may set it according to your system.
The second and third lines are for rendering two different files and different frames.
You may save the above code in a text file and save it anywhere in your computer and with any name but extension should be .bat
Now right click on file (.bat) we just created and Run as administrator
You may add as much files and frames as you like
or have several interesting and useful render options here Command Line