Braley, J.
The refusal of the president to answer the eleventh interrogatory affords no ground of appeal. The defendant was not compelled under R. L. c. 173, §' 63, to disclose in advance its theory of the accident, or to state the facts derived from investigation, upon which it relied to establish its defense. Gunn v. New York, New Haven, & Hartford Railroad, 171 Mass. 417. Robbins v. Brockton Street Railway, 180 Mass. 51. Spinney v. Boston Elevated Railway, 188 Mass. 30.
*533Nor was there any error at the trial in the rulings upon the admission and exclusion of evidence. It was within the discretion of the presiding judge, which does not appear to have been wrongly exercised, to exclude the testimony of the plaintiff’s medical expert, upon the ground, that in his opinion the witness lacked sufficient medical experience. Muskeget Island Club v. Nantucket, 185 Mass. 303. Lakeside Manuf. Co. v. Worcester, 186 Mass. 552. Moreover, as his evidence if admitted bore only on the measure of damages, the jury having found on the issue of negligence that there was no liability, the plaintiff was not prejudiced. The exclusion of the letter also was discretionary, as it contains only by remote inference, if at all, any possible allusion to the plaintiff. Jennings v. Rooney, 183 Mass. 577. Robinson v. Old Colony Street Railway, 189 Mass. 594. The defendant, without objection or exception, having put a hypothetical question to an expert called by it to give his opinion as to the causes by which the car might have been derailed, the plaintiff asked that his answer be excluded, because it assumed the existence of facts not in evidence, and which the jury could not fairly find to have been true. To a refusal to exclude this answer the plaintiff excepted. Williams v. Clarke, 182 Mass. 316. A hypothetical question rests upon either assumed facts already in evidence, or assumed facts which may be put in evidence. In determining the scope, fulness and distinctness of the questions, much must be left to the discretion of the presiding judge, which ought not to be overridden, unless it very clearly appears to have been wrongly exercised. Chalmers v. Whitmore Manuf. Co. 164 Mass. 532, 533. Anderson v. Albertstamm, 176 Mass. 87. Commonwealth v. Johnson, 188 Mass. 382, 384, 385, 386. By the testimony of other witnesses the defendant had laid a proper foundation upon which to rest the assumption of facts in the question asked. The witness assumed in his answer’, as the question itself was predicated upon such assumption, that the car track and the switch were apparently in good condition as well after as before the accident, and then proceeded to give his opinion, that, if these conditions were found to have existed at *534the time, the tongue or the switch might have- been moved a little when the forward trucks passed over, and, if this occurred, the rear trucks as they followed might be caught, causing the car partially to leave the track. In further demonstrating how this might happen, his statements, that if dirt had worked into the switch, the tongue might have been pushed out from the rail on which the car was travelling, causing it to run off the track, or, if the switch tongue had become slightly worn, it would be a little low, causing the tread of the wheel to lift from the rail as it passed over, may be regarded either as additional reasons for his opinion, derived from experience, or as other possible consistent explanations, falling within the scope of the inquiry.
We pass to the rulings requested by the plaintiff, and to the instructions under which the case went to the jury. In all there were twelve requests. Of these, the second, third, fifth, eighth and ninth were given substantially in the language requested, while the seventh, subject to the plaintiff’s exception, was given in a modified form. The tenth was properly refused, as the judge was not called upon to single out a portion of the evidence for comment. Besides, the jury must clearly have understood from the instructions, which if not in terms certainly in substance embodied the request, that the credibility of the witnesses, and the weight of the evidence as to any adequate explanation offered by the defendant, were all for their determination. The' twelfth also was properly refused for the first reason given above for the refusal of the tenth. By the first part of the eleventh request, the plaintiff directed the attention of the judge to the degree of care required of a common carrier of passengers, and the instructions were in conformity therewith. The second part could not properly be given as it subjected the defendant to a greater liability than the law imposes. Millmore v. Boston Elevated Railway, 194 Mass. 323. Marshall v. Boston & Worcester Street Railway, 195 Mass. 284, 287.
But the plaintiff’s principal complaint arises from the refusal to give the sixth and seventh requests, without modification. In the case of Ware v. Gay, 11 Pick. 106, 112, where a stage coach, in which the plaintiff was a passenger, overturned and broke his leg, it was said, “ The wheel came off upon a plain and good *535level road, without coming in contact with any other object. This evidence made a prima facie case for the plaintiff. ... We are of the opinion that the law would imply negligence from those facts. It would result from them, that the coach was not properly fitted and provided. Then the burden of proof would change, and it would be for the defendants to rebut that legal inference.” This form of statement of the law both as to the presumption of negligence on the part of the carrier, where the injury to the passenger is of such a nature, that the accident according to common experience would not have happened if there had not been a defect in the road or the equipment by which it is operated, and the burden of proof after a prima facie case has been made out, is found in very many of the cases where the subject has been considered. Fairchild v. California Stage Co. 13 Cal. 599. Osgood v. Los Angeles Traction Co. 137 Cal. 280. Derwort v. Loomer, 21 Conn. 245. Yonge v. Kinney, 28 Ga. 111. New York, Chicago & St. Louis Railroad v. Blumenthal, 160 Ill. 40. Pittsburgh, Cincinnati & St. Louis Railroad v. Williams, 74 Ind. 462. Southern Kansas Railroad v. Walsh, 45 Kans. 653, 659. Louisville & Portland Railroad v. Smith, 2 Duv. (Ky.) 556. Baltimore & Ohio Railroad v. Worthington, 21 Md. 275. Stevens v. E. & N. A. Railroad, 66 Maine, 74, 77. Stoody v. Detroit, Grand Rapids & Western Railroad, 124 Mich. 420. McLean v. Burbank, 11 Minn. 277. Sawyer v. Hannibal & St. Joseph Railroad, 37 Mo. 240. Curtis v. Rochester & Syracuse Railroad, 18 N. Y. 534. Caldwell v. New Jersey Steamboat Co. 47 N. Y. 282. Iron Railroad v. Mowery, 36 Ohio St. 418, 422. Sullivan v. Philadelphia & Reading Railroad, 30 Penn. St. 234. Boss v. Providence & Worcester Railroad, 15 R. I. 149, 154. Zemp v. Wilmington & Manchester Railroad, 9 Rich. (S. C.) 84. Baltimore & Ohio Railroad v. Wrightman, 29 Gratt. 431. Stokes v. Saltonstall, 13 Pet. 181. New Jersey Railroad v. Pollard, 22 Wall. 341. Gleeson v. Virginia Midland Railroad, 140 U. S. 435. Skinner v. London, Brighton & South Coast Railroad, 5 Exch. 787. Dawson v. Manchester, Sheffield & Lincolnshire Railway, 7 H. & N. 1037. Carpue v. London & Brighton Railway, 5 Q. B. 747. Kearney v. London, Brighton & South Coast Railroad, L. R. 6 Q. B. 759, 762. In some of our more recent decisions, the presumption standing alone is stated to *536be sufficient to support an inference of negligence, unless the defendant, by going forward with the evidence, offers what the jury may find to be an adequate or satisfactory explanation. Le Barron v. East Boston Ferry Co. 11 Allen, 312, 316, 317. Feital v. Middlesex Railroad, 109 Mass. 398. Joy v. Winnisimmet Co. 114 Mass. 63. White v. Boston & Albany Railroad, 144 Mass. 404. Griffin v. Boston & Albany Railroad, 148 Mass. 143, 146, 147. Cassady v. Old Colony Street Railway, 184 Mass. 156, 162. Hebblethwaite v. Old Colony Street Railway, 192 Mass. 295. Egan v. Old Colony Street Railway, 195 Mass. 159. Minihan v. Boston Elevated Railway, 197 Mass. 367. But whichever form of expression may be chosen, prima facie evidence in legal intendment means evidence which if unrebutted or unexplained is sufficient to maintain the proposition and “ warrant the conclusion to support which it is introduced.” Emmons v. Westfield Bank, 97 Mass. 230, 243. Crane v. Morris, 6 Pet. 598, 611. A prima fade case, when made out, does not however, either necessarily or usually, change the burden of proof. It stands only until the contrary is shown. Commonwealth v. Kimball, 24 Pick. 359, 365. Wilder v. Cowles, 100 Mass. 487. The distinction between the burden of proof and the weight or preponderance of the evidence is sometimes overlooked. In the sense of the burden of the evidence, the burden of proof may change from one side to the other as the trial proceeds, but in the sense of maintaining the issue involved in the action it constantly remains on the party alleging the fact which constitutes the issue, and, when all the evidence has been introduced, the jury must say whether it has been maintained. Central Bridge Co. v. Butler, 2 Gray, 130, 132. It is in this sense that the phrase has been employed, or impliedly understood, in the class of cases in our reports to which the case at bar belongs. The defendant in the explanation which it offered was not called upon to account satisfactorily for the accident, although oftentimes when this has been done the presumption of the carrier’s carelessness disappears, but only to show or explain that it had not been guilty of negligence. After it had introduced evidence from which the jury could find that it had used due care in the construction, equipment and maintenance of the railway, the burden of proof had not been shifted but still remained upon the plaintiff to *537establish the defendant’s negligence upon all the evidence, of which the presumption or inference of negligence upon proof of the derailment and injury formed only a part.
The requests, therefore, except so far as given, were properly refused, and the instructions, which fully and accurately recognized this distinction, were correct in law.
Exceptions overruled.