Complex sentence: The tunnel is the first stage of a high-speed rail connection between Steinkjer and Trondheim, but because the tunnel is being built with curves with too small diameter, the section will not be suitable for high-speed trains.
Split into simpler sentences: The tunnel is the first stage of a high-speed rail connection between Steinkjer and Trondheim. Because the tunnel is being built with curves with too small diameter, the section will not be suitable for high-speed trains.