https://github.com/railwayapp/cli/blob/master/src/commands/scale.rs
Think you can use the Railway CLI to modify the amount of replicas

On that, the CLI interacts via graphql so you are able to do it that way too

Thanks a lot for your suggestions!

I checked and indeed can be manually triggered from the CLI. But is it possible to do it automatically?

There is no first party way to do this right now. You would have to set up an automation with the graphql public api

Unfortunately, I could not find any similar implementation, documentation or example to scale replicas.

Do you have any info for the same?

Pepper has linked the source code for the scale command above.

But it is not yet clear for me how to practically use it…

The code shows you how to use the API to scale up or down a Railway service. You can take that code, rewrite it into a language you are comfortable with, mix in your own business logic, and you will eventually come to the solution you want.

You can probably spin up a secondary service in Railway where it only calls the CLI/GraphQL endpointse periodically based on cron jobs

And here comes my point. Which is the endpoint for scaling?

I could not find any reference…

The CLI is open source; you can see what GQL calls it makes and to where.