Hi Ruud. I was concerned about one aspect of Your Connections and I will try to explain.
When connectig servos this should only be done via a bord made for servos like the "ESU switch pilot servo" or the Board from DigiRails that Robert V O use in his video on the railcrosing-servos.
(I do not know if the Arduino is made for servos)
Inside the servo you have a circuitboard and a positionsensor that is dependent on Control by a special unit and will not function like a clean DC motor unless you remove the sensor and Board.
This Board is allso supersensetive to interferance and this makes the servo jitter unless it is controlled the way intended.
If the controlling-Device is exposed of interferance this will allso make the servo jitter.
I am simply writing this because I can not see how it is wired. You can try to move the Arduino further away from most cables to see if this helps.
As a general rule if the servos have the same itemnumber and are not the same, this Points in the direction of a very cheap manufacturer With bad quality.
For servos that will last a long time, buy Futaba, Graupner, or Hitec. The cheapest servo I have is PowerHD 1711MG but although good Power one of the 15 I have, rusted stuck after two years of no use.
The problem is simply that a 3 Euro servo will last one year, a 30Euro Futaba servo will last forever while dealing With 10 times the abuse. (The 1711MG is around 10Euro)
But if you have Connected servos to one common and the two other wires to the Arduino this is a big NO simply because the controlfunction is lost and the chances of interferance increase.
It is possible to operate the servo as a DC motor but I would not do this unless it was 1 servo for one special task..., (instructions on Youtube) Very timeconsuming.
Btw I think the fact you have kept the spring in the turnouts is the best option yo avoid constant preassure on the
servo
So if all servowires are Connected to the Arduino it is most likely the way you have placed the wires that needs a "cleanup" - keep servowires away from the bus and powervires.
Best of Luck and Regards.
JC