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.
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.
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.