As the resident civil engineer, Greg is officially in charge of building and maintaining the train track. Now that Mary can pull up on the train table, she thinks she should be part of the engineering action too, greatly complicating Greg's ability to execute this particular familial duty. For Mary, "civil engineering" means pulling things apart and slobbering on them. The last time Greg tried to build a track, I heard repeated shouts of "Agh, Mary!" and I think he finally had to halt construction until after baby bedtime.
Here is a picture of Greg and his apprenticed engineer (who couldn't be happier):
