How are the Maghrib and Isha prayers performed when traveling a distance of 250 km, departing before Maghrib and arriving after Isha?

It is permissible for you to combine the Maghrib and 'Isha prayers by delaying Maghrib, as long as you are traveling. This is based on the hadith of Ibn 'Abbas, that the Prophet, peace and blessings be upon him, "If the time for Maghrib came upon him while he was at home, he would combine it with 'Isha. But if its time had not come upon him while he was at home, he would ride until 'Isha, then dismount and combine them."