Can one buy a bus pass in the U.S. similar to the the rail pass. What is the difference in bus & rail fares?
Be aware that except for going between Barcelona and Madrid you cannot ride Eurolines buses solely within one country. You must start in one country and end in another.
Bus fares are cheaper than standard fares for long distance rides on high-speed trains. However, if you book well in advance on a country's national rail site, you often can get very good discount fares.
The same is true for Thalys trains between Amsterdam and Paris. The standard 2nd class fare is 123 EUR. Booked well in advance (up to 90 days allowed) at www.thalys.com, you can get a Smoove fare as low as 35 EUR.