The speed limit will depend upon the track conditions and whether the track is operated by the major railways (CN and CP) or by a shortline operator.
A shortline operator will put very little time and effort into maintaining the track. They don't need to as they can take their time picking up and delivering freight cars the short distances (relatively speaking) to the industry.
However, the majors are running time-sensitive intermodal freights which need to arrive on time. Otherwise they will be charged penalties. So, the majors invest a lot of time and money into track maintenance.
While there are no section crews that maintained a 5 - 10 mile chunk of track, the track today is maintained in "better condition" than in previous years. It's just that everything has gone high-tech. For example, CP inspects their track between Toronto and Montreal every 3 months using a special train pulled by a GP9. The train consists of 3 stainless steel cars. The last car has a large "bay window" with two "operators" looking at each rail. Each operator has a computer in front of them. As they see defects in the roadbed and trackwork, a click of the mouse automatically notes the mileage and problem. At the end of the run, the information is given to the roadmaster responsible for the subdivision who then makes sure the work is carried out.
Then there's the Sperry Rail Services vehicles that regularly patrol the rails. The SRS self-propelled cars (and their new 10 ton Hy-Rail trucks) do a "magnetic imaging" of every section of rail. This magnetic imaging detects cracks and flaws in the rails so that the rail can be outchanged.
And there are track maintenance crews stationed at Division points or at key places along the way. However, they no longer travel to the job on putt-putt speeders as in days gone by. And they no longer wield spike mauls. They travel along the highway in their hy-rail 10 ton trucks. The trucks have hydraulic booms to lift ties and rails. The spikes are driven in by hydraulic spike hammers that look like a jack hammer. The bolts are tightened by hydraulic-driven impact guns. The rails are cut by motorized cutoff saws that resemble a chain saw with a 14" carborundrum blade.
Ties are replaced by specialized crews and equipment that operate out of Toronto and Ottawa. The equipment consists of Fairmont-Harsco hydraulic tie removers, tie inserters, and spikers. If you are interested in buying some of this equipment used, visit the CP Rail Asset Disposal website.
https://www8.cpr.ca/inet62/snp/Pages/ViewTender.aspx?Tender=1611#Images
https://www8.cpr.ca/inet62/snp/Pages/ViewTender.aspx?Tender=1612#Images
https://www8.cpr.ca/inet62/snp/Pages/ViewTender.aspx?Tender=1608#Images
Two additional pieces of equipment consists of the hydraulic ballast tamper that tamps the ballast underneath the tie, making sure that the transition over the section of track is smooth, that the ties are level between rails, or have the proper super-elevation on curves. The ballast regulator then "trims" the ballast so that the crushed stone is flush with the tops of the ties and properly sloped on the sides of the roadbed.
The ballast tamper and ballast regulator also operate alone retamping and re-levelling the track.
So, it all depends on which railway line and railway you're looking at. And, just because a few spikes might have worked loose doesn't mean that the rails are about to spread apart. It takes a lot to move those rails.
And it also depends on the "philosophy" and money that each railway puts into track maintenance. In the case of CP Rail, their track maintenance budget has been over $150 million a year. Quite a chunk of change, eh!?
Bob M.