Mrs. M's train is stationary. Molly & Mr. M approach.

For her, the distance from Maple to Dixon is less than the  proper length of her train, while her husband's train is even shorter. As we shall see, the relativistic length of anything that is moving decreases as the velocity approaches the velocity of light.

From Mrs. Modern's perspective, the light reflected from the caboose and locomotive of her own train reaches her eye simultaneously -- assuming she is standing in the exact center of the train. Her husband's train is something else again. First light reaches his caboose, while it is still traveling to catch up with his engine.

Then light reaches her own engine and caboose simultaneously.

Finally, light reaches his locomotive.

Thus simultaneity is a matter of perspective.

Now this lack of simultaneity between the caboose and engine of the other train,  will be observed by all the folks on Mrs. M's train. Everyone in her frame of reference, in the world of his moving train, can agree on what is meant by "at the same time." The engineer in the locomotive, the break man in the caboose, and the conductor collecting tickets somewhere in between can all agree on the proper time in their frame of reference. Their clocks can be synchronized.

They may see the light at different times but that is not the celebrated time desynchronization of Einstein's relativity. What they see is desynchronization due to signal transmission delay time. That kind of desynchronization was understood long before Einstein, and they can account for it. That is the difference between observing and measuring.

So while only sees her engine and caboose leave burn marks simultaneously if she happens to be the same distance from each of them, she can would measure the same result even if the distances were different. We will see that the engineer in the locomotive and the brakeman in the caboose can measure their burn marks as being at the simultaneous in proper time even if their observed time is different. To measure, he could compare observations using identical clocks at different locations. These identical clocks must, however, be at rest relative to each other. 

Moving clocks, however, are a different story.