I've witnessed this phenomenon: Why are only the tie tips dry? Why are only the stones around the tips dry? This is a railroad in Ukraine in the morning after a rain at night. Around 80% of observed tips looked like that, slightly elevated tips were not dry.
$\begingroup$ Vibration causes them to heat up slightly? $\endgroup$– int_uaOct 30, 2020 at 9:51
3$\begingroup$ @int_ua less about heat and more that the water is "shaken off", like a dog :) $\endgroup$– monoceresOct 30, 2020 at 10:52
Figure 1. A concrete sleeper mould with reinforcing steel. Concrete sleepers are reinforced with pre-stressed steel. Image source: BFT-International.
The sleepers will act as heat sinks during the day soaking up heat from the ground. At night this heat is let off. The steel will conduct heat to the ends of the sleepers warming the end-plates and supplying more heat through the lower thermal resistance than at other points on the surface of the sleeper. The steel in the phots seems to have end plates which will improve the end-heating significantly.
Engineering Toolbox gives the following conductivity values:
Concrete 1.0 to 1.8 W/mK Steel 36 to 54 W/mK
The steel is a far superior heat conductor than the concrete.
$\begingroup$ Transistor, I am fascinated with your prediction about steel end plates! Can I somehow test if it's indeed the case? Magnets? Or, perhaps more simply, how to find out the sleeper's name or serial number to look how it's made? $\endgroup$ Oct 30, 2020 at 21:56
$\begingroup$ Also, I observed that some tips, those that are not as much burried in gravel, were not dry at all. No steel plates there? Sorry, did not make a photo of those. $\endgroup$ Oct 30, 2020 at 22:06
$\begingroup$ I worked as a signal engineer many decades ago so I was aware of concrete sleeper manufacture but not in detail. I know they were tipped out of the moulds in some ridiculously short time - a minute, I think. I found that image after a web search and I don't know how typical the end plates are. I suspect that they might not exist in most sleepers. As an aside, when we bake potatoes at home we use a steel skewer to bring heat into the centre. I suspect that it helps quite a bit. I could experiment but who wants to waste a good potato? $\endgroup$ Oct 30, 2020 at 22:18
$\begingroup$ google.com/… the most common sleeper in my place is the one on the pictures. You can see steel rods at the tips but I can't tell from the blueprints whether the plates are in place. Can you? As for potatoes, I can't but agree: if it ain't broken, don't fix it. $\endgroup$ Oct 31, 2020 at 13:05
1$\begingroup$ No, I couldn't find any better information. I'd say that even simple rods would cause the effect. $\endgroup$ Oct 31, 2020 at 13:36
They have collected heat during the day . And being solid they conduct heat better than the loose gravel. So they collect more heat and when cool the heat is conducted back to the surface better than the loose gravel. Under some humidity and temperature conditions the few degrees of temperature causes faster evaporation from the solid concrete. You sometime see the same affect on streets after a light rain sprinkle; grass is wet and any pavements are dry.
1$\begingroup$ Why would this cause the tips to dry first? Why would it cause some of the nearby rocks (not touching the sleeper) to dry before the centre of the sleeper (solidly connected to the ends)? $\endgroup$ Oct 30, 2020 at 15:55