Learn French: Le voyage en train est souvent fatigant.

This is a special spelling rule for this lesson! When used as a pure adjective describing a noun (a "verbal adjective"), verbs ending in -guer drop the "u". If it were used as an active verb expressing an action (a true present participle, like "tiring out the children"), it would keep the "u" and be spelled fatiguant. Since it describes the noun Le voyage here, it is the adjective fatigant.

If fatigant acts as an adjective here, does it have to agree with the noun?

Yes! Because it functions as a verbal adjective describing Le voyage (a masculine singular noun), it takes the masculine singular form: fatigant. If we were describing a feminine noun, like La journée (the day), it would become fatigante. Remember: true present participles never change their endings, but verbal adjectives must agree with the noun they modify.

Why do we say en train instead of par train or dans le train?

When talking about a mode of transportation in a general way (like "by train"), French uses the preposition en for closed vehicles you ride inside (en train, en voiture, en avion). You would use dans le train if you were talking about a specific physical location, such as "I left my bag in the train" (J'ai laissé mon sac dans le train).

Why does the adverb souvent come directly after the verb est?