Complex sentence: CART and IRL ran alongside each other for over a decade until the CART series, having changed name to the Champ Car World Series went bankrupt, and merged into the IRL, which rebranded as the IndyCar series.
Split into simpler sentences: CART and IRL ran alongside each other for over a decade until the CART series, having changed name to the Champ Car World Series went bankrupt. It merged into the IRL, which rebranded as the IndyCar series.