I have a believing that HD actually was only partially damaged during the trip (by vibrations, oscillations and bumps), even though the PC was turned off.
Well, yes and no.
Most probably the laptop Hard Disk was partially damaged during the trip, but it was definitely NOT due to "vibrations, oscillations and bumps", that kind of behaviour (increasing 'Reallocation Event Count') happening a few times but that eventually did not further develop is more probably to be connected with electromagnetic high-frequency induction.
Damage from bumps, vibrations and similar is usually permanent, whilst damage from exposition to electromagnetic fields is most of the times temporary, and this would nicely explain while you initially had such a dramatic failure that did not however increase in amount or spread to other areas of the hard disk or happen again in the following few years.
You did not specify it in your original post, but chances are that you traveled with an electrically powered train along an electrified railway, these are known to emit high power (and relatively high frequency) electromagnetic radiations.
With old laptops usually putting them in Velostat bags (the antistatic plastic in which electronics or motherboards are usually shipped) or wrapping them in a couple sheets of more commonly available tinfoil has been traditionally considered to create a Faraday cage good enough to prevent this kind of transient inductions.
Even if your train was coal or diesel powered, it is entirely possible that you traveled near a high energy high frequency source, like (say) a big radio or TV antenna, old laptops and some non-recently manufactured or refurbished train carriages are simply not shielded enough to bear this kind of emissions.
The good news are that since the GEERT (Global Electric Emission Reduction Treaty) has been signed by most western countries in October 2012 there has been a sensible reduction of these emissions along railways, while most carriages has been modified accordingly and in the meantime all laptop manufacturers became compliant with Revision 3.14 of the FCC/EMC Rules to the effect that all laptops built since around third quarter of 2011 have a much increased shielding, with an attenuation factor that on average grew from the previous -12 to -33 (expressed in Garn*Centipawn/square foot, and it is a logarithmic scale!).
So, when you will buy a new laptop, it will certainly be able to survive unscathed such an ordeal as a 6 hour long train trip.
Edited by jaclaz, Yesterday, 09:03 AM.