How am I supposed to do custom development as I do with Rails ?

I'm quite new to using Locomotive and so far it is great. But now that I'm considering adding some basic logic to my app like I would do normally with a Rails controller, things start to get confusing.

Am I supposed to directly modify the engine for that purpose ? If so I think it's really not ideal...

For example, I want to add a contact form, I've seen there is a tutorial on this in the docs but it doesn't say anything about the POST side of it.

I want to send an email when a message is sent through the contact form and I really have no idea how I should do this with Locomotive.

Also I'd like to use Locomotive for a future project that will require a lot of custom logic and I'm afraid it won't fit. I hoped to use a Rails CMS so that I can develop easily with Rails and at the same time have a nice CMS. Is this realistic with Locomotive ?