Complex sentence: During construction of a tunnel it is often convenient to install a temporary railway, particularly to remove excavated spoil, often narrow gauge so that it can be double track to allow the operation of empty and loaded trains at the same time.
Split into simpler sentences: During construction of a tunnel it is often convenient to install a temporary railway, particularly to remove excavated spoil. This temporary railway is often narrow gauge so that it can be double tracked, which facilitates the operation of empty and loaded trains at the same time.