'High Voltage' is a big step up from the previous DLC, and a great addition to the game. For your money you get a lot more than in the previous DLC. Here's a quick breakdown:
New map(s): We're in Switzerland this time and the level of detail in the map is a huge step up from the previous DLC. There's quite a bit of variety as your trains travel along the shores of lakes, through alpine forests, and of course up and down the mountains themselves. You can also choose to play free games on much smaller chunks of the main map, which is a good innovation that gives a nice option for shorter games. The music is genuinely quite catchy.
Ten new engines: All ten electric locomotives are beautifully modelled and look and sound great. There are 3 European specific, 3 for North America, and the other 4 are shared between the two. You can also use these electric locomotives in the other maps when playing on free mode, which isn't true of the other DLC locomotives. They really do look amazing.
2 new scenarios: The gameplay here doesn't introduce many differences compared to the base game, but the electrified lines feel a bit different to build, and the news reports covering events from the 1920s add a bit of flavour. If you liked the base game, you'll definitely enjoy these. Also, you are able to use 7 of the 10 new engines in the scenarios, which is nice!
New mechanic - Electrification: Practically this doesn't really change how you play the game, except that you have to click a button to build electrified lines instead of regular ones. In theory, this isn't a massive change, but building an electric railway has a very different feel to it.
For me, this is possibly the best RE2 DLC, and I'd strongly recommend it. Also, respect to the devs for the sheer number of electricity based puns.