Answer:
Here are the parts of speech for the underlined words in each sentence:
1. We took an express train.
- "express" is an adjective modifying "train."
2. A train whistle sounded in the distance.
- "train" is a noun, and "whistle" is a noun.
3. I will train your dog.
- "train" is a verb in this sentence.
4. Cross the street on a green light.
- "green" is an adjective describing the type of "light."
5. Turn right at the next cross street.
- "cross" is a noun in this sentence, and "next" is an adjective describing "cross street."