Instruments have improved and involved over the years that now all instruments for the inspections are compact and small enough that can be installed on regular revenue passenger trains. Now one shinkansen train will be fitted with the Instruments and inspections will be done during regular scheduled Shinkansen run.

The example of how Instruments have evolved, one of the Instruments I use as a chemist is Mass Spectroscopy and when I was in a graduate school this Instrument occupied the entire room of the size of a large conference room. Now Mass Spectroscopy is the size of desktop computer and is far faster and far more advanced in analysis then when I was in a graduate school.

P.S.
There are two yellow inspection trains called "Dr. Yellow." This inspection cars are officially called Shinkansen Denki-kidou Sougou-Shikensha 923 (Shinkansen Series 923 train, 新幹線電気軌道総合試験車923形). One is called T4 Configuration which belongs to JR Tokai, and another almost identical one (also yellow in color) is called T5 Configuration which belongs to JR West.

The one which has retired this January is T4 Configuration which belongs to JR Tokai. T5 Configuration which belongs to JR West will continue to run least till 2027. You can still be possible to see "Dr. Yellow" running on a track. JR West has not said the exact retirement date of T5 Configuration, JR West has said possible to run T5 Configuration beyond 2027. Also, JR West said they have not finalized the inspection plan after the retirement of T5 Configuration.

JR East has an inspection car called E936 Series (nicknamed East-i) which runs on Tohoku and Jyoetsu Shinkansen. It is not yellow in color, and nobody seems to give a damn about E936 where "Dr. Yellow" enjoys celebrity status.

JR East E936