Train Sim World 2020

I recommend just doing all the services you want to do and this will come along the way. But if you get bored because the services in this DLC can be fairly repetitive, I recommend using the first scenario, California Commuter. You get ~5700xp every 22 minutes (~15k xp/hr) as well around 12 miles toward the 400 you need in the F40PH. The entire route has a high speed limit so you can just go full throttle the entire time between stops, plus the length is a good balance of xp/hr and not risking losing an hour and a half's worth of xp because you weren't paying attention and blew threw a red signal.

This will not help you gain progress toward the 100 services achievement, but there's a service you can repeat every 2-3 minutes that is better for that. I would wait to repeat California Commuter until you're done with everything else though because you will gain xp for everything else you do so ideally this will be the last achievement you get for this DLC unless you really like doing all of the services available instead of repeating short ones.