Is Yichud permitted in an elevator?
Some Poskim rule the prohibition of Yichud applies in an elevator, and it is therefore forbidden to be alone in an elevator with a woman even for a very short amount of time. Majority of Poskim however rule the prohibition of Yichud does not apply to elevators. Practically, one may follow this ruling, especially in a time of need, such as one needs to get to work and the like. Nonetheless, despite the above it is a Middas Chassidus to avoid Yichud in an elevator even if it will only be for a short amount of time.
Express elevator: An express elevator climbs many floors at once without having stops in-between, and is commonly built in high rise buildings, to allow for the quick transfer of people from lower to upper floors, and vice versa. Express elevators cause a Yichud concern being it is not possible to stop the elevator for quite some time, in which one remains in seclusion with the opposite gender. Practically, the ruling is as follows: Express elevators that take less than 35 seconds until they reach their first possible destination, do not have Yichud laws applicable to it. If they remain more than 35 seconds, but less than five minutes, the elevator may be used in a time of need. The average express elevator in a tall building takes between one to two minutes to reach its first possible destination. Accordingly, it is permitted for a person to enter the elevator in a time of need, even if it causes one to be in seclusion with the opposite gender for that duration of time.
Late at night or when people are not around: Some Poskim write that the above allowance to be in Yichud in an elevator only applies at times that people are around, and it is thus probable that the elevator will be stopped at any moment, as in such a case one has the Heter of Pesach Pasuch Lereshus Harabim is applicable. If, however, it is late at night, or another time that people are not around, then one to be stringent not to be in Yichud in an elevator. [However, in truth, from many of the previous Poskim it is evident that even this is allowed in a time of need, as it is not possible to remain Shiur Tuma in the elevator without the door opening, and as soon as the door opens the Yichud is broken even late at night, as one is now in view of a public area. Hence, seemingly one may be lenient even in this, in a time of need. If, however, in truth it was established that people are no longer in the building at this time of night, then not only would using the elevator together be forbidden, but even being in the building together alone with the opposite gender would be forbidden.]
Is there a minimum time limit for the prohibition of Yichud?
Some Poskim rule there is no minimum measurement of time for the prohibition of Yichud, and therefore, it is forbidden to be in Yichud with the opposite gender, even for a mere moment, without having a Yichud breaker [i.e. Pesach Pasuach to public; Husband in city; three women; two Kosher men] available. Other Poskim however rule the prohibition of Yichud only applies if the pair will remain together for the time of “Shiur Tuma” which is discussed regarding a Sotah. Practically, if it is possible to remain in seclusion for Shiur Tuma, then even if one decides to stay in seclusion for less than Shiur Tuma, it is prohibited. [For example, it is forbidden for one to lock his office door and remain in seclusion with a woman even if he plans to open it within less time than Shiur Tuma.] If, however, it is not possible to remain in Halachic Yichud for Shiur Tuma, then it is permitted to remain together for less than Shiur Tuma.
How much is Shiur Tuma regarding a Sotah and regarding Yichud?
Shiur Tuma is the amount of time the couple need to engage in marital intercourse. The Talmud records various opinions regarding the amount of time that is defined as Shiur Tuma and the Poskim rule that it is the amount of time it takes to fry an egg and eat it. Some Poskim rule Shiur Tuma is between 5-8 minutes. Other Poskim rule Shiur Tuma is 35 seconds.
 See Minchas Yitzchok 4/94-97; Taharas Yom Tov 7; Nitei Gavriel 15/1
 Maharil Diskin in understanding of Minchas Yitzchak 4/94-98; Shevet Halevi 3/182 [in contradiction to his previous response in 2/131, and even this response contains inconsistencies in logic, as brought in Minchas Yitzchak 4/96, as he agrees that if it’s not possible to have Yichud for Shiur Tuma then its Mutar, so why does he prohibit elevators!]
 The reason: As the Sages in the Talmud, Rambam and Shulchan Aruch did not make mention of any minimum Shiur for Yichud, and hence we can conclude that being in Yichud for even a short amount of time, less than Shiur Tuma, is forbidden. This is because Yichud is considered part of Lo Sikrav, and is hence forbidden even if it can’t lead to marital intercourse. [Shevet Halevi ibid]
 Igros Moshe E.H. 4/65-16; Chelkas Yaakov 2/14; Minchas Yitzchok 4/94-97 [Mutar in time of need, such as needs to get to work etc]; Many leading Rabbonim brought in Minchas Yitzchak 4/97 and Taharas Yom Tov 7, including: Rav Yelles; Betzel Hachochma; Rav Shternbuch; Shevet Halevi 2/131 [unlike his later response in 3/182]; Mishneh Halachos 4/187; Tzitz Eliezer 6/22-4; Nitei Gavriel 15/1
 The reason: As the prohibition of Yichud does not apply for less than Shiur Tuma and the elevator may be stopped by any person before the man/woman are able to remain in seclusion for the minimum prohibited time of Yichud. [Poskim ibid] Furthermore, it is not possible for the elevator doors to remain closed for Shiur Tuma, even if other people are not around, and hence even an express elevator is permitted. [See Minchas Yitzchak ibid]
 Minchas Yitzchak ibid only permits this in a time of need as perhaps according to the ruling of Maharil Diskin even this is forbidden; See also Rabbeinu Yerucham 1; Sefer Hayireh of Rabbeinu Yona 237; Maharsham 2 Mafteichos for 76; Nitei Gavriel 15/6
 Dvar Halacha Teshuvah 11
 Minchas Yitzchak ibid based on that Shiur Tuma is at least five minutes.
Other opinions: Some Poskim rule that if one will remain more than 35 seconds in the elevator it contains a prohibition of Yichud, as 35 seconds is Shiur Tuma. [Dvar Halacha ibid]
 Elevators normally take 1.5 seconds between floors, and hence in a 100 floor building, it would take about 2.5 minutes to reach the first possible destination. Sears tower [Chicago] elevator takes about 60 seconds from bottom to top. [Seen in video] The new world trade center elevator takes less than 45 seconds from bottom to top, traveling at a speed of up to 2000 feet per minute [per article in NY Times]
 See Chelkas Yaakov 2/14 who implies the Heter is due to Pesach Pasuach; Dvar Halacha 15; Nitei Gavriel 15/2-4 in name of Rav of Debritzin, and so writes Rav Dubov in “The laws of Yichud” 144; Ohel Yaakov 136 in name of Poskim
 This ruling of the Nitei Gavriel ibid and others is extremely puzzling, as the Heter is not due to Pesach Pasuach, but due to the fact they don’t remain Shiur Tuma, as explicit in the words of the Minchas Yitzchak ibid and other Poskim. According to the Nitei Gavriel that late at night in a building lobby or hallway is not considered Pesach Pasuach then it should be forbidden to even be alone in the building with the woman, let alone be alone in the elevator!
 See Minchas Yitzchok 4/94-97; Taharas Yom Tov 7; Nitei Gavriel 15/1
 Maharil Diskin; Shevet Halevi 3/182 sides with his opinion, however learns him to only be stringent in a case that it is possible to remain in seclusion for Shiur Tuma; See also Shevet Halevi 2/131
 The reason: As Chatzi Shiur is forbidden Biblically. Alternatively, due to worry that they may decide to stay for longer. [Maharil Diskin] As the Sages in the Talmud, Rambam and Shulchan Aruch did not make mention of any minimum Shiur for Yichud, and hence we can conclude that being in Yichud for even a short amount of time, less than Shiur Tuma, is forbidden. This is because Yichud is considered part of Lo Sikrav, and is hence forbidden even if it can’t lead to marital intercourse. [Shevet Halevi ibid] As the Sages suspected for hugging and kissing, which can occur with less than Shiur Tuma.
 Imrei Eish E.H. 107; Igros Moshe E.H. 4/65-16; Shevet Halevi 2/131 [unlike his later response in 3/182]; See Noda Beyehuda E.H. 160 that by a Sotah, Setira for less than Shiur Tuma is not called Setira [however see Minchas Yitzchok 4/94-97 that perhaps by Issur Yichud the Noda Beyehuda would agree to the prohibition in even less than Shiur Tuma]
 The reason: As the Sages did not suspect for hugging and kissing in Yichud, and only suspected for actual relations, which can only happen if they remain secluded for Shiur Tuma. [Igros Moshe ibid]
 Igros Moshe E.H. 4/65-16 [see there that it is forbidden due to Chashad]; Minchas Yitzchok 4/95 and Shevet Halevi 2/131; 3/182 [that so is according to Maharil Diskin, while according to the Mahram Eish even in this case it is permitted, however they conclude that we rule like Maharam Diskin]
 Shevet Halevi 2/131; 3/182 and Tzitz Eliezer 6/22-4 that so applies according to all opinions, even according to Maharil Diskin; Minchas Yitzchak 4/94-98 [Only permits this in a time of need as perhaps according to the ruling of Maharil Diskin even this is forbidden]
 See in great length Minchas Yitzchok 4/94-97
 Sotah 41
 Sotah ibid
 Michaber E.H. 178/4
 Minchas Yitzchok 4/94
 Dvar Halacha Teshuvah 11