Before this line was converted to become part on the Line C of the Simvliet Metro, this was part of the Railway network, the same cannot be said for the Line D as far west as Zeerwijk which was also once the railway, although freight trains still operate there.
As far as all concerned, the next station past Poort as a Line D of Simvliet Metro is Rijndam Airport. However, back in pre-Metro days and when MSB was Beta 3.14, Poort was served by diesel trains. At the time, tunnel north of Poort station was a mystery.
Of course, this is when we had Beta 3.14. The line was not electrified due to the restricted clearances, and there was a lack of proper railway equipment, leaving the use of 6000 series locomotive with borrowed RET stock being the only option. The repaintable MG2 stock could have been used behind a 6000 locomotive, disguising from any form of RET (as shown above). However in later years, the Class 230 DMUs were used.
There are few suggestions considered beyond Poort station when it was part of the Railway network:
1) There was a turnback siding inside the tunnel, to allow trains to switch to the other side of the tracks for their return journey;
2) The line continues north somewhere past Simvilet Airport which has since been closed;
3) The line continued to Simvilet Airport that was part of the railway, like it did on the Rijndam Airport Line out of Nooderpoort (prior to 2006);
4) Poort was a terminus. Switching points was provided south of Poort station.
You can tell us what this line would have looked prior to conversion to metro from Beta 3.15 onwards.