The data-set JTRAIN2 was provided with: 

Wooldridge, Jeffrey M. (200x), Introductory Econometrics: A Modern Approach, x. Edition, South Western College Publishing, Mason (Ohio). 

(Note: x stands for different years/editions)

Jeffrey M. Wooldridge cites the original source as follows:

"LaLonde, Robert J. (1986), 'Evaluating the Econometric Evaluations of Training Programs with Experimental Data', American Economic 
Review 76(4), 604–620. Professor Jeff Biddle, at MSU, kindly passed the data along to me. He obtained it from Professor Lalonde."

These data contain the following variables: 

Obs:     445

 1. train                             =1 if assigned to job training
 2. age                               age in 1977
 3. educ                              years of education
 4. black                             =1 if black
 5. hisp                              =1 if Hispanic
 6. married                           =1 if married
 7. nodegree                          =1 if no high school degree
 8. mosinex                           # mnths prior to 1/78 in expmnt
 9. re74                              real earns., 1974, $1000s
10. re75                              real earns., 1975, $1000s
11. re78                              real earns., 1978, $1000s
12. unem74                            =1 if unem. all of 1974
13. unem75                            =1 if unem. all of 1975
14. unem78                            =1 if unem. all of 1978
15. lre74                             log(re74); zero if re74 == 0
16. lre75                             log(re75); zero if re75 == 0
17. lre78                             log(re78); zero if re78 == 0
18. agesq                             age^2
19. mostrn                            months in training