Are the Dhuhr and Asr prayers valid, given that the traveler intended to combine them with an early combination (jam' taqdim), then completed Dhuhr and prayed Asr at its due time because the train moved?

Your intention to combine prayers by performing them earlier (jama' taqdeem), followed by not actually doing so and instead praying each prayer at its appointed time, does not affect the validity of the two prayers. Your obligation for them has been fulfilled, and nothing further is required of you.