Why are sidings usully lower than the main line?
mudchicken will deliver a definitive answer, I think.
One reason is that, up until recently, it was easier when reballasting main lines to leave much of the old ballast in the same place, which has the effect over time of raising the ballast prism. You don’t bother with the materials or manpower expense for sidings.
Another reason can be drainage – you may also have noticed that on four-track mains the ‘inside iron’ can be raised above the outer tracks. This gives better gravity drainage and can reduce the cost of any drainage piping or vaults under the track structure. But I think the previous answer is far more likely to be the reason…
I think Overmod is correct. The ballast and surfacing work is targeted to the main and the sidings get less attention and less often. It makes perfectly good sense because most of the tonnage wil be on the main and the main is usually higher speed than the siding. His point on drainage is also correct.
Mac
I believe the main line being higher than sidings is the constant ballasting. I have seen a picture of Pennsylvania’s Horseshoe curve where tha track foreman is standing on the siding with the main line being 4 feet higher. The caption said it was ballast buildup over time.
Just to add a bit to the above … sidings also have lighter rail which adds to the aparent lower level
(1) Drainage, keep the most important track the highest & hopefully driest in signal areas this also helps eliminate shunts with water being a poor conductor of DC current in the rails…
(2) Grade protection…keep errant cars from rolling out or fouling the main track. Also helps stop trains rolling at very low speed. (and creates headaches climbing back out of the hole)
(3) Snowdrift & blowing dirt/ “snirt” protection (esp in the southwest and up north)
(4) Around at-grade crossings, the relationship of track grades is tied to the crossing grade and vice versa. This really becomes an exercise in trigonometry when there are curves with healthy super-elevation in them.
The two tracks cannot change grades until they get off the long switch ties behind the frog.You do not start changing grades with a vertical curve until you are off the switch ties. (Amazing how many non-railroad civil engineers and surveyors cannot figure this out!) The side track more often then not cannot run the same speed as the main line