If someone really wants to delve into it, they can do much more than just challenge log. As long as you know to focus light, when you hit ele 25, the relic grind will go much quicker. Start a PF for your level at 25+. You will likely still hit ele 35 and have way more than 500 crystals before finishing your light. I cannot speak to how it looks if you focus hitting ele 35, but maybe Vaer can give you an idea.

You can ride the train, if there is one. However, things will take much longer, and you will have to grind light anyway. You will also be removing light grinding locations away from yourself, since you out-leveled them. The only advantages to the train exp push are logistical. For some people, they are the advantages that they feel they need. So that depends on the person.