Go to this part of the map and locate the jump onto the train tracks.
There's a glitch where the cops can't make the jump to follow you onto the train tracks. You can start a pursuit, jump on the tracks, and evade the pursuit by driving out of the pursuit radius.
You can also increase your heat level by parking on top of the tracks where you can still be seen. The cops can see you and continue to increase your heat, but they won't be able to bust you.
For The Heat is Off achievement, you need to escape at each individual heat level (one through six) and it will unlock. You'll likely have already escaped lower heat levels if you've already been playing the game awhile, so it's probably faster to start at 6 and work your way down. So start a pursuit, get to heat level 6 and escape. Start a new pursuit, get to heat 5, and escape. Repeat for heat 4-1 until it unlocks.
For this achievement, you need to do a minimum of six pursuits. In each one, you must hit a peak of each of the six levels. One pursuit, you must hit heat level six. Then another one, heat level five. Go on down the level and you will be rewarded with this confusing achievement. I can 100% confirm this.