Strasburg uses diesel power on special trips during the Thomas the Tank Engine events. They also run the gas (or is it diesel?) motorcar during the week during the off-season, with the regular steam train on the weekend. I've seen a few instances where the crowd out numbers the seats in the motor car and the diesel and coaches are used.
I wouldn't say 100% of Strasburg's public operations are steam powered at all. I don't believe there are any, except the Colorado narrow gauge and Cass that are truly 100% steam powered.
I remember when the Mt. Washington Cog Railway was 100% steam powered - I rode it then. I'll be back there in two weeks for the first ride (but not first visit) since it went to nearly 100% diesel power.