I would recommend a quality boot. Many suck up the price on a pair of Danner's or Red Wing. Steel toes are required when working on bridges but I wouldn't work without steel toes.
The height of the boot must be a minimum of 6", measured from the sole. An 8" boot provides better ankle support when walking on ballast. Here is a snippet of something I recently received from NS in regard to proper footwear that may help you. Note the bold area. This differs from the NS rules for agreement workers but the rest is the same. Pay close attention to the heel angle:
[3] Protective footwear:
[3.1] Upper portion must be constructed of leather or leather-like material that provides firm ankle support and fits snugly about the foot, ankle and leg. Footwear must be 6 inches minimum in height as measured from the bottom of the heel to the top of the uppers.@ Soles and heels must be sturdy, non-leather and in good condition. Boots need to have a steel-toe.
[3.2] Must have a defined heel of no less than a 45 degree angle to the walking surface with a minimum height of 3/8 inches as measured between the bottom of the heel and the bottom of the sole.
It is recommended that employees whose duties regularly require ascending/ descending ladders, poles, structures and/ or mounting/ dismounting rail equipment wear protective footwear having a 90 degree heel as measured vertically between the bottom of the heel and the bottom of the sole.