May a Kohen (that is, a member of the priestly clan, who are prohibited by the Torah from exposing themselves to dead bodies) ride on a train that passes over, through, or under a cemetery? May he ride a train that has traversed one?
See שו"ת שואל ומשיב מהדו"ג ח"ב סי' מג, who addresses this issue--seemingly there is a problem of אהל זרוק לא שמיה אהל, which would mean that the train would not "interrupt" the טומאה. However, the שואל ומשיב says that something which is made to move would be considered an אהל, in which case the train would be block the טומאה.
The חזון איש יו"ד סי' ריא סק"ח holds that train cars are considered an אהל זרוק.
However, even if we accept the שואל ומשיב's position, there may be another problem because the train is made out of metal, in which case it should not block the טומאה--as the חזון איש points out in ס"ק ט.
R. Tzvi Hirsch Orenstein (ברכת רצ"ה סי' יב) quotes R. Yitzchak Elchanan Spektor as permitting כהנים to travel on a train above a graveyard, on condition that two other leading poskim accept his position. R. Orenstein himself apparently does not accept this position. R. David Tzvi Hoffman (מלמד להועיל יו"ד סי' קלג) cites this dispute, and rules leniently under certain conditions.
Finally, the ציץ אליעזר חי"ב סי' סב discusses this issue at length, עיי"ש.
There are 4 questions here: 1) over, 2) through, 3) under and 4) a train that has traversed a cemetery
The Kohen's Handbook by Rabbi Yochanan-Alexander-Lombard says on page 63:
An object that is not susceptible to tuma and is not a person or a utensil and is not resting on one, blocks tuma from spreading through it. ….
I guess that a railway track going over a cemetery, would use a viaduct (see “Over Land”) which would block the tuma. So 1) over: would probably not be a problem.
2) through: page 57 says that “a kohen could come into contact with tumas-mes by touching, carrying or roof” (all defined later in the book). Touching and carrying are unlikely to be problems. For roof see later. There is a rabbinic prohibition of coming within 4 amos (6.5 to 7.5 feet) of a grave. IMHO this is also unlikely.
3) under: I do not know.
4) a train that has traversed a cemetery: For aeroplanes, I have not seen (I know this is not a proof) any prohibition of taking an aeroplane that has traversed a cemetery and therefore deduce that there should probably be no problem with a train that has traversed a cemetery.
Related to “roof”, the book says on page 109, “A kohen may not fly or travel in any vehicle over Jewish graves, because a moving vehicle does not block tuma, particularly if it is made of metal …..”
As ever, CY LOR who has experience in these matters.