On Greek suburban railway (proastiakos) I noticed that on stations have an internal rail as long as the train platform:
I was wondering what it the use for this internal rail? Also the suburban railway is electric and uses overhead lines and not a third rail layout.
The rail does not extend beyond the platform (I think it is even a bit shorter).
At least here in Germany such additional rails are used in sensitive areas, like stations, bridges etc., to prevent extensive damage in case of a derailment. If a car were to derail away from the platform (in your case), the third rail would still hold the car and would prevent it from drifting too far apart (and probably overturning etc.).
Here is such an example:
Führungsschienen auf einer Brücke. Source: Wikipedia
This is called a Guard rail. These are placed in areas with restricted clearance to prevent excessive damage in case of derailment. In this case, it prevents a derailed train from hitting the platform where passengers are standing and/or other passenger trains.
It's also used by a switch to allow passenger trains to reach a platform, while freight trains just hiball through without using the switch, giving them more clearance. Here's an example, from the Westside Express Service of Tri-Met in Metro Portland, Oregon, which is operated by the Portland and Western line, at the Tualatin station. See https://imgur.com/gallery/31nSCZw
The internal rail is there to make it possible for narrow gauge freight trains to pass through the station if needed. This was mostly essential in the 1960's but is still used today on suburban rail routes which share track with mainline trains or freight trains.
