Tubes could be as big as buses. I think a lot of people like and enjoy the comfort of traveling on a train, whether it be compartments or regular seats. I know I do.
First off, the terms "tubes" and "capsules" may be a little confusing. Tubes are analogous to a road, capsules are like a car (or bus) that operates on a road.
While ET3 tubes could be build big enough to accommodate bus sized capsules, such a scheme has already been proven in the market to be of lower value. If it were true that "a lot of people like and enjoy the comforts of traveling on a train" trains would still remain the predominate way of travel between cities as they were in the year 1910 in the US. In 1910 90% of Americans traveled between major city by train, now it is less than 1%. About 10% fly, and bus use is accounts for about 3% of US passenger miles. The vast majority of people prefer to travel by automobile. If we are to displace cars, ET3 must offer even more value than cars. Value = benefit / cost. For trips over 20 miles, ET3 is much faster (benefit) for less than a tenth the cost of driving of flying.
The first railroads/trains in the US were build privately and according to the needs of the owners -- so they were built using a gage (track width and rail/wheel size) that suited the economics of the particular route. Later it was realized that if all built to the same standard gage, that trains could operate on a NETWORK of tracks to great advantage. This avoided a lot of unnecessary unloading and loading of different size trains. If ET3 is to eventually networked together into a national (and international) system, it must all be built to the same tube diameter standard.
Much of the advantage of large vehicles (e.g. trains compared with muscle powered wagons) was in labor savings. A few men could operate a train consisting of dozens of cars (or one driver a bus with several dozen passengers). ET3 uses automation techniques to reduce labor cost to a much greater degree than possible with large vehicles.
The cost of ET3 is very sensitive to the tube diameter. If the capsules were made "as big as a bus" it would increase the cost by a factor of about 30. If a fare for one way from SF to LA (350 miles) in a car size ET3 system were $20 (to produce a favorable ROI for owners), the fare for a bus sized system would be $600 (to produce the same ROI at the same number of passengers per day use factor). If the two systems were built side-by-side, the car sized system would attract most of the passengers, so the bus sized system would not attract enough passengers to recover the cost. This is why passenger train use in the US failed in by the 1950s -- passenger trains only survive due to gross government subsidy where government (tax payers) suffer over 90% of the total cost.
The most important thing to get correct with ET3 is the capsule diameter. If the diameter is a little too small ET3 will not achieve sufficient utility to carry most cargo and passenger comfort would suffer. If a little too big, the cost prevents maximum use. Our considerable research into this topic of optimal size indicates that the optimal capsule diameter is 1.3m (51") and the corresponding tube diameter is 1.5m (60"). Smaller or larger sizes are possible for highly specialized "off network" use, but we strongly advocate use of the optimum (standard) size for all licensees because of the extreme added value of ET3 if the network is expanded on a global basis.