No, mi:100 means mile 100.

Railroad mileposts are whole numbers. I'm not sure where you picked up the habit of appending ".0" to the value displayed on the actual signs, but it's not necessary to add that to mileposts when they are always signed as whole numbers. Railroad features other than mileposts often have linear referencing positions in tenths and hundredths of miles, but mileposts by definition do not. 