Sore fingers are a pain even in larger OO rail joiners!
Answer is to make a home made fishplate (rail joiner) fitting tool. Take a piece of spare rail about 6" inches (150mm) long and with a pair of pliers bend one end of the rail upwards to a sort of Hockey stick shape. Note bend is top to bottom not sideways on the rail. The bent end needs to allow around 50% of a joiner to sit on its end. So trim its length as required. File rail end to allow new joiners to slide on easy. Push a cork or similar onto the far end to protect the hands.
Now simply slide on a new joiner onto the foot of the homemade tool and push the joiner onto the rail it is to be fitted onto. Hold joiner in place and slide off the tool.
Don't forget to file a chamfer on the Streamline rail ends too for an easier fit.
Rail Fishplate tool.jpg