I am the bridge that connects both sides.
I am the railroad that travels across the nation.
I speak two languages, crossing words back and forth.
What root word am I?

am is the root word for-  the word am I?

Explanation:

The Latin root word am means “love.”  

A root word is a word or  part of a word can be form the basis of new words through the addition of prefixes and suffixes.

am is a prefix word

Examples:  amateur, amatory, and "amenity"

Here, the word "amenity" expresses the word "am i"

amenity means things that people "love" to have that make a place comfortable to live in.

bridge and railroad helps people to adopt a comfortable and ease of living.

Hence ., am is the root word for-  the word am I?