SUrfacing keeps the roadbed uniformly supportave of the tract structure. Unless track is tamped (surfaced) every six months or so, low stops develop, resulting in unsupported portions and ties that "pump" up and down.
This pumping action weakens the tie and degrades the ride quality. It causes joint movement and often causes joint bars or their bolts to break. Stresses are transfered to the ties with a hard subsurface, greatly amplyfing the loads on those ties.
With concrete ties, the flexing of rails in extremely damaging, as concrete does not take well to flexing without developing fissures. Also, the pumping ties erode the bottom surface of the concrete, creating a destructive abrasive slurry that wears away at the tie.
When one concrete tie goes unsupported or becomes damaged, its loads are transfered to the neighboring ties along with the distructive flexing.
Surfacing is part of the normal maintience of main track. The manufacturers of concrete ties stress its importance to the point of denying warranty work when track is not properly surfaced.
Oh yes, I did fail to state that the manufacturer is replacing the ties three-for-two in an effort to help defray costs.
Hope that answers your question, Bob...
And, eh Dave, we're both showing our age. I believe southern yellow pine would have been be much better, just as the Pennsy deemed after it tried concrete in the 1930's.
Clem.