In terms of literal meaning, they are equivalent. The difference is one of emphasis.
The first reason to use the past continuous is to emphasize the main event of some story. If several things were happening, placing the main one in the past simple and the rest in past perfect serves to emphasize the main event. For example:
I was walking down the street--listening to music--and suddenly this guy jumped out at me!
Experience of Continuous Action
The other main reason one might use past continuous is to draw attention to the experience of an ongoing event. The past can be used to emphasize the experience of an ongoing event. This is a more subtle difference.
To give a practical example:
- I was working in the field yesterday.
- I worked in the field yesterday.
To my ear, (1) brings to mind the action and experience of field work: standing in the field, under the sun, etc. The speaker experienced the action "working in the field." (2) lacks such associations; to me (2) is more clear and businesslike: yesterday the speaker did work in the field.
- I was trying as hard as I could to lift it.
- I tried as hard as I could to lift it.
As above, (1) brings to mind someone straining against the load, red in the face, etc. They are in the middle of trying. (2) is more matter-of-fact. The speaker tried as hard as he/she could, but now it is over.